Unmute get field mapping test (#12422)

This test was muted due to an [intermediate state][1] during type
removal. It now passes per my local testing.

[1]: #2440 (comment)

Signed-off-by: Andrew Ross <andrross@amazon.com>

rest-api-spec/src/main/resources/rest-api-spec/test/indices.get_field_mapping/20_missing_field.yml

@@ -1,9 +1,5 @@
 ---
 "Return empty object if field doesn't exist, but index does":
-  - skip:
-      version: "all"
-      reason: "AwaitsFix https://github.com/opensearch-project/OpenSearch/issues/2440"
-
   - do:
         indices.create:
           index: test_index
@@ -18,7 +14,5 @@
       indices.get_field_mapping:
         index: test_index
         fields: not_existent
-        ignore: 404    # ignore 404 failures for now
-                       # see: https://github.com/opensearch-project/OpenSearch/issues/2440
 
   - match: { 'test_index.mappings': {}}