Drop ClickTracking integration from mw.user.bucket

Support will be dropped per https://gerrit.wikimedia.org/r/#/c/46205/

The tracking has no functional implications & I don't really see any point in tracking the bucketing decisions (which are both at 100/0)

Change-Id: I920e8719c372fe3bac1003060411ca03a726a753
Merge "Drop ClickTracking integration from mw.user.bucket"