Remove vestigal references to SYNC replication command

We've ripped pretty much all of this out: let's remove the remains.

synapse/replication/tcp/commands.py

@@ -289,14 +289,6 @@ class FederationAckCommand(Command):
         return str(self.token)
 
 
-class SyncCommand(Command):
-    """Used for testing. The client protocol implementation allows waiting
-    on a SYNC command with a specified data.
-    """
-
-    NAME = "SYNC"
-
-
 class RemovePusherCommand(Command):
     """Sent by the client to request the master remove the given pusher.
 
@@ -419,7 +411,6 @@ _COMMANDS = (
     ReplicateCommand,
     UserSyncCommand,
     FederationAckCommand,
-    SyncCommand,
     RemovePusherCommand,
     InvalidateCacheCommand,
     UserIpCommand,
@@ -437,7 +428,6 @@ VALID_SERVER_COMMANDS = (
     PositionCommand.NAME,
     ErrorCommand.NAME,
     PingCommand.NAME,
-    SyncCommand.NAME,
     RemoteServerUpCommand.NAME,
 )