remove HomeServer.get_config (#9815)

Every single time I want to access the config object, I have to remember
whether or not we use `get_config`. Let's just get rid of it.

synapse/crypto/keyring.py

@@ -501,7 +501,7 @@ class StoreKeyFetcher(KeyFetcher):
 class BaseV2KeyFetcher(KeyFetcher):
     def __init__(self, hs: "HomeServer"):
         self.store = hs.get_datastore()
-        self.config = hs.get_config()
+        self.config = hs.config
 
     async def process_v2_response(
         self, from_server: str, response_json: JsonDict, time_added_ms: int