Remove ab-testing.js file

The naive manner in which it uses _gaq causes failures in the jasmine 
tests, and it seems to not be in use since the commit 
93d71e1d2231599ab21bfd6f58e5fe3172bfdfb0 that removed the JS from the
view (if not from the repo).
Merge pull request #545 from alphagov/remove_ab_breakage

Remove ab-testing.js file