Cleanup the isc_<*>mgr_createinc() constructors

Cleanup the isc_<*>mgr_createinc() constructors

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 3388ef36b3cff409f78db59cc1c5d71839423ee0)