sstate-cache-management: rm_by_stamps - remove .siginfo and .done files

sstate-cache-management: rm_by_stamps - remove .siginfo and .done files

* it's possible that corresponding .tgz files were already removed
  (e.g. with -d option and older version of this script) and this
  won't find orphaned .siginfo or .done files to remove
* add sort -u to count files found multiple times only once

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>