locks: remove changelog comments

This is only of historical interest, and anyone interested in the
history can dig out an old version of locks.c from from git.

Triggered by the observation that it references the now-removed
Documentation/filesystems/mandatory-locking.rst.

Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>