Clean up get_backend_from_dcb

Replace the original version of the function with the reference version
and use it everywhere. Added runtime assertions to check that an invalid
DCB is never processed.

server/modules/routing/readwritesplit/rwsplit_internal.hh

@@ -60,7 +60,6 @@ bool send_readonly_error(DCB *dcb);
  * The following are implemented in readwritesplit.c
  */
 int router_handle_state_switch(DCB *dcb, DCB_REASON reason, void *data);
-SRWBackend get_backend_from_dcb(RWSplitSession *rses, DCB *dcb);
 int rses_get_max_replication_lag(RWSplitSession *rses);
 
 /*