Update service/tags_manager.php for postgres compatibility

When installing the extension on a board using postgres (tested on phpbb 3.1.2 w/ postgres 9.3) the update query using the table alias in the SET clause caused a query error.

In the scope where "t.count" is available, there is only one table in play, thus the table identifier "t" is unnecessary as the column "count" is not ambiguous.
Merge pull request #38 from Octonami/patch-1

Update service/tags_manager.php for postgres compatibility