Remove manual transactions when deleting tags.

Handling the transactions manually creates the possiblity
of unclosed transactions as reported here:
http://askbot.org/en/question/11230/errors-while-trying-to-rename-or-delete-unused-tags/

However, there's no need to do this manual handling; the simplest
solution is to just remove the decorator.

Also added tests.
Merge pull request #160 from powlo/fix_delete_unused_tags

Remove manual transactions when deleting tags.