Improve deleting of the last item in kill-ring.

When deleting the last item in the *Kill Ring* buffer, delete the
preceding separator.  The buffer display is now consistent after
deleting items from the buffer.
Merge branch 'delete-last' of github.com:T-J-Teru/browse-kill-ring into T-J-Teru-delete-last
Merge branch 'T-J-Teru-delete-last'