CLEANUP: connection: merge CO_FL_NOTIFY_DATA and CO_FL_NOTIFY_DONE

Both flags became equal in commit 82967bf9 ("MINOR: connection: adjust
CO_FL_NOTIFY_DATA after removal of flags"), which already predicted the
overlap between xprt_done_cb() and wake() after the removal of the DATA
specific flags in 1.8. Let's simply remove CO_FL_NOTIFY_DATA since the
"_DONE" version already covers everything and explains the intent well
enough.
2 files changed