Remove CLIENT.oper_by_my, Client_SetOperByMe() and Client_OperByMe()

Remove CLIENT.oper_by_my, Client_SetOperByMe() and Client_OperByMe()

All places where Client_OperByMe() is used can either be converted to
Client_HasMode(Client, 'o') or Op_Check().

And Op_Check() itself can use the connection handle for deciding whether
the IRC Operator is a local user or not.