LU-17379 ptlrpc: fix check for callback discard

LU-17379 ptlrpc: fix check for callback discard

In ptlrpc_unregister_reply() decision about need to
discard request-out callback is done too early, before
LNetMDUnlink() invokes reply callback. Therefore at the
monent of discard check rq_reply_unlinked is not set yet
and discard is skipped always.

Patch removes discard check from __ptlrpc_cli_wait_unlink()
and does that after LNetMDUnlink() call right inside
ptlrpc_unregister_reply().
That makes __ptlrpc_cli_wait_unlink() unused, so it was
removed and only ptlrpc_cli_wait_unlink() remains

Fixes: babf0232273 ("LU-13368 lnet: discard the callback")
Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: I6448cafa8a0b81d7ba0172ad1709e75e592d4924
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53937
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>