Fix false debug assertion

Given the following query:

PREPARE ps FROM 'PREPARE ps2 FROM \'SELECT 1\'';

The debug assertion is hit even though this is valid, albeit unsupported,
SQL. An optimization would be to ignore the query if the prepared
statement type is another prepared statement.

server/core/queryclassifier.cc

@@ -148,8 +148,6 @@ uint32_t get_prepare_type(GWBUF* buffer)
         }
     }
 
-    mxb_assert((type & (QUERY_TYPE_PREPARE_STMT | QUERY_TYPE_PREPARE_NAMED_STMT)) == 0);
-
     return type;
 }