nfsd4: remove obselete comment

[ Upstream commit 80479eb8 ]

Mandatory locking has been removed.  And the rest of this comment is
redundant with the code.

Reported-by: Jeff layton <jlayton@kernel.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>