Cleanup the isc_<*>mgr_createinc() constructors (v9.16)

Previously, the taskmgr, timermgr and socketmgr had a constructor variant, that would create the mgr on top of existing appctx. This was no longer true and isc_<>mgr was just calling isc_<>mgr_create() directly without any extra code.

This commit just cleans up the extra function.

(cherry picked from commit 3388ef36)