Don't assume postgres tables are in the public schema during db port

When fetching the list of tables from the postgres database during the
db port, it is assumed that the tables are in the public schema. This is
not always the case, so lets just rely on postgres to determine the
default schema to use.

scripts/synapse_port_db

@@ -447,9 +447,7 @@ class Porter(object):
 
             postgres_tables = yield self.postgres_store._simple_select_onecol(
                 table="information_schema.tables",
-                keyvalues={
-                    "table_schema": "public",
-                },
+                keyvalues={},
                 retcol="distinct table_name",
             )