924349: fix selinux warnings

removed restorecon /var/cache/thumbslug and changed semanage -a to -d in postun.

Thumbslug makes no use of /var/cache/thumbslug. I suspect this was a copy paste error from the example SELinux packaging drafts:

http://fedoraproject.org/wiki/PackagingDrafts/SELinux 

where it makes reference to /var/cache/myapp

the uninstall post script was trying to ADD the thumbslug port instead of deleting it. The -d option seems to follow the SELinux packaging guideline for uninstall post scripts.