Use a kill-buffer-hook to ensure that the preview is removed.

Previously, if the `*Kill Ring*' buffer is killed in by calling
`kill-buffer' directly, rather than `browse-kill-ring-quit' then the
preview is left active.  After this patch the preview will be removed
even in this case.
Merge branch 'cleanup-on-kill' of github.com:T-J-Teru/browse-kill-ring into T-J-Teru-cleanup-on-kill
Merge branch 'T-J-Teru-cleanup-on-kill'