MXS-2359 Remove special handling for SHOW TABLES

The code only handled the basic version of the command, returning incorrect
results if modifiers were used. The code is now removed, causing the command
to be routed to the backend of the current database. This will give correct
results as long as that backend contains all the tables of the database e.g.
no table sharding.

server/modules/routing/schemarouter/schemaroutersession.hh

@@ -144,7 +144,6 @@ private:
     /** Shard mapping functions */
     void                 send_databases();
     bool                 send_shards();
-    bool                 send_tables(GWBUF* pPacket);
     void                 query_databases();
     int                  inspect_mapping_states(SSRBackend& bref, GWBUF** wbuf);
     enum showdb_response parse_mapping_response(SSRBackend& bref, GWBUF** buffer);