sstate-cache-management: Fix remove_duplicated() to work after SSTATE_SWSPEC change

* format of filenames for sstate archives was changed in:
  commit 6f823a23c5f1d0ffa0a27db1c1bc1907de788505
  Author: Richard Purdie <richard.purdie@linuxfoundation.org>
  Date:   Fri Dec 20 12:06:02 2013 +0000
  Subject: sstate: Convert to use ':' as a filename sperator and use SSTATE_SWSPEC globally
* remove_duplicated() wasn't able to find available architectures and
  duplicate files since this change
* add extra step to remove old sstate archives starting with sstate-
  (instead of sstate:)