cifs: fix unlinking of rename target when server doesn't support open file renames

The patch to make cifs_rename undoable broke renaming one file on top of
another when the server doesn't support busy file renames. Remove the
code that uses busy file renames to unlink the target file, and just
have it call cifs_unlink. If the rename of the source file fails, then
the unlink won't be undoable, but hopefully that's rare enough that it
won't be a problem.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>