filtering/QueryParser: remove distinct

Removing distinct improves performance of some queries by a factor of 3 (for example query "machine_tag ~= 'value'" on TestResult table). Execution time of most of the queries is unchanged.

I've created this MR to open discussion about that distinct, it's added to every user query which can be expensive in some cases. @mupuf do you remember why it was there in the first place (8866c68f)? I cannot think of any existing query that would benefit from it, maybe there is. If there is such query we could add distinct just to that one.