remove extension existence helper

This is only used in one place.

src/nominatim_db/db/connection.py

@@ -175,20 +175,13 @@ class Connection(psycopg2.extensions.connection):
         return (int(version_parts[0]), int(version_parts[1]))
 
 
-    def extension_loaded(self, extension_name: str) -> bool:
-        """ Return True if the hstore extension is loaded in the database.
-        """
-        with self.cursor() as cur:
-            cur.execute('SELECT extname FROM pg_extension WHERE extname = %s', (extension_name, ))
-            return cur.rowcount > 0
-
-
 class ConnectionContext(ContextManager[Connection]):
     """ Context manager of the connection that also provides direct access
         to the underlying connection.
     """
     connection: Connection
 
+
 def connect(dsn: str) -> ConnectionContext:
     """ Open a connection to the database using the specialised connection
         factory. The returned object may be used in conjunction with 'with'.