Remove GatewayPrx and IDeletePrx

After some time (Gateway, 2011 in b50bcdd6; IDelete, 2012 in 36133da1) both of these services are ready to be removed for 5.1 and thereby **significantly** reduce the deprecation warnings caused by the Ice generated code.

Regular build and test status should suffice, though a close look at the changes is probably worthwhile. Other deprecations will be removed later by @jburel.