wiki: Remove DeletePage RPC
The Wiki service has functional duplication with the OperationService RPCs. This means that if the clients are refactored, the wiki RPCs can be removed. This change removes the DeletePage RPC as the client that was using it, is no more: gitlab!57106 (merged).
The proto is removed too, as the client is unable to call the RPC and doesn't use it anymore.
Edited by Zeger-Jan van de Weg