sstate-cache-management.sh: Fix rm_by_stamps() to work after SSTATE_SWSPEC change

sstate-cache-management.sh: Fix rm_by_stamps() 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
* this one doesn't need special care for old sstate- names
  they will be removed automatically as they don't match with
  any checksum in rigth format from stamps directory

(From OE-Core rev: aa36f9c9b5abac58de899f98803d1c4375678044)

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