Remove delete_bookmark tearDown 

I've identified this code as playing a part in some of the b2g crashes which result in broken pipe errors. I don't think this is the only source of the crash put it is perhaps one of them.

The crash in question is logged here:
https://bugzilla.mozilla.org/show_bug.cgi?id=827264
and if you want to replicate it you can do so by running the old version of test_browser_bookmark.py with `repeat=50` .. after about 10-20 loops b2g will crash.
Without this tearDown it is stable.