history clearing does not clear trends_uint

problem most likely also present in 1.8 branch

"clear history" button in item properties does not clear trends_uint.
in ./include/items.inc.php:1411 trends table is cleared, but trend_uint is not - probably just a matter of adding a new line.

interestingly, both trends and trends_uint will be cleared if item is deleted, thus using housekeeper (maybe fixed with ZBX-1365 ?)