Remove format.atom check.

The check was added at
https://github.com/rubygems/rubygems.org/commit/3d0c24e27ada9989a252e998c19d623d1307e1a2#diff-e4663ec6bcde373fc1ee998eb69725b1R3
Commit message says it was added to make clearance happy, however we
dont need that check anymore.

review @dwradcliffe @qrush 

also this was not fully working under rails 4+ , so removing it would make it easier to upgrade.