Remove more references to get_datastore

These have snuck in since #12031 was started.

Also a couple of other cleanups while we're in the area.

changelog.d/12067.feature

@@ -0,0 +1 @@
+Implement experimental support for [MSC3720](https://github.com/matrix-org/matrix-doc/pull/3720) (account status endpoints).

synapse/handlers/account.py

@@ -23,7 +23,7 @@
 
 class AccountHandler:
     def __init__(self, hs: "HomeServer"):
-        self._store = hs.get_datastore()
+        self._main_store = hs.get_datastores().main
         self._is_mine = hs.is_mine
         self._federation_client = hs.get_federation_client()
 
@@ -98,7 +98,7 @@ async def _get_local_account_status(self, user_id: UserID) -> JsonDict:
         """
         status = {"exists": False}
 
-        userinfo = await self._store.get_userinfo_by_id(user_id.to_string())
+        userinfo = await self._main_store.get_userinfo_by_id(user_id.to_string())
 
         if userinfo is not None:
             status = {

synapse/rest/client/account.py

@@ -904,9 +904,6 @@ class AccountStatusRestServlet(RestServlet):
     def __init__(self, hs: "HomeServer"):
         super().__init__()
         self._auth = hs.get_auth()
-        self._store = hs.get_datastore()
-        self._is_mine = hs.is_mine
-        self._federation_client = hs.get_federation_client()
         self._account_handler = hs.get_account_handler()
 
     async def on_POST(self, request: SynapseRequest) -> Tuple[int, JsonDict]: