Revert "Fix potential double free of ratchet identity key"

This reverts commit 3b099e9.

This resulted in:
```
==5285== Invalid read of size 16
==5285==    at 0x4FA80FC: ec_public_key_serialize (in /usr/lib64/libsignal-protocol-c.so.2.3.3)
==5285==    by 0x4E5E76: omemo_identity_key (omemo.c:419)
==5285==    by 0x4EBB7E: omemo_bundle_publish (omemo.c:129)
==5285==    by 0x4E5BD9: omemo_publish_crypto_materials (omemo.c:335)
==5285==    by 0x460407: sv_ev_connection_features_received (server_events.c:202)
==5285==    by 0x43AA87: connection_features_received (connection.c:779)
==5285==    by 0x4418C9: _disco_info_response_id_handler_onconnect (iq.c:2423)
==5285==    by 0x43B9F1: _iq_handler (iq.c:241)
==5285==    by 0x5163848: ??? (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x516A224: ??? (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x5E4FE43: ??? (in /usr/lib64/libxml2.so.2.12.8)
==5285==    by 0x5E54927: xmlParseChunk (in /usr/lib64/libxml2.so.2.12.8)
==5285==    by 0x5163450: xmpp_run_once (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x439797: connection_check_events (connection.c:162)
==5285==    by 0x43894E: session_process_events (session.c:256)
==5285==    by 0x4319FF: prof_run (profanity.c:128)
==5285==    by 0x4EDAE6: main (main.c:174)
==5285==  Address 0xa1cb1e0 is 16 bytes inside a block of size 72 free'd
==5285==    at 0x484875B: free (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==5285==    by 0x4395D0: _xfree (connection.c:110)
==5285==    by 0x516A1A7: xmpp_stanza_release (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x516A16C: xmpp_stanza_release (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x516A16C: xmpp_stanza_release (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x516A230: ??? (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x5E4FE43: ??? (in /usr/lib64/libxml2.so.2.12.8)
==5285==    by 0x5E54927: xmlParseChunk (in /usr/lib64/libxml2.so.2.12.8)
==5285==    by 0x5163450: xmpp_run_once (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x439797: connection_check_events (connection.c:162)
==5285==    by 0x43894E: session_process_events (session.c:256)
==5285==    by 0x4319FF: prof_run (profanity.c:128)
==5285==    by 0x4EDAE6: main (main.c:174)
==5285==  Block was alloc'd at
==5285==    at 0x4845794: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==5285==    by 0x43958A: _xmalloc (connection.c:102)
==5285==    by 0x516A0D1: xmpp_stanza_new (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x516BF34: ??? (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x5F10A17: ??? (in /usr/lib64/libxml2.so.2.12.8)
==5285==    by 0x5E5481F: xmlParseChunk (in /usr/lib64/libxml2.so.2.12.8)
==5285==    by 0x5163450: xmpp_run_once (in /usr/lib64/libstrophe.so.0.13.1)
==5285==    by 0x439797: connection_check_events (connection.c:162)
==5285==    by 0x43894E: session_process_events (session.c:256)
==5285==    by 0x4319FF: prof_run (profanity.c:128)
==5285==    by 0x4EDAE6: main (main.c:174)
```

Tested via sending OMEMO messages via 1:1 and in MUC.

src/omemo/omemo.c

@@ -271,7 +271,11 @@ omemo_on_disconnect(void)
     free_keyfile(&omemo_ctx.identity);
 
     signal_protocol_store_context_destroy(omemo_ctx.store);
+    ec_public_key* pub = ratchet_identity_key_pair_get_public(omemo_ctx.identity_key_pair);
+    ec_private_key* priv = ratchet_identity_key_pair_get_private(omemo_ctx.identity_key_pair);
     ratchet_identity_key_pair_destroy((signal_type_base*)omemo_ctx.identity_key_pair);
+    ec_private_key_destroy((signal_type_base*)priv);
+    ec_public_key_destroy((signal_type_base*)pub);
 
     signal_context_destroy(omemo_ctx.signal);
     memset(&omemo_ctx, 0, sizeof(omemo_ctx));
@@ -1466,8 +1470,6 @@ _load_identity(void)
     ec_private_key* private_key;
     curve_decode_private_point(&private_key, identity_key_private, identity_key_private_len, omemo_ctx.signal);
     ratchet_identity_key_pair_create(&omemo_ctx.identity_key_pair, public_key, private_key);
-    ec_private_key_destroy((signal_type_base*)private_key);
-    ec_public_key_destroy((signal_type_base*)public_key);
 
     char** keys = NULL;
     int i;