Revert "Implement /builds?branches=true, which works just like /branches"

This reverts commit 70fb4a957a34b99ffb51ffcae15aca7837866b0e.

After more discussion about the problem that I would like to fix with
this commit (travis-ci/travis-web#123), we decided to change the
implementation in travis-api. This will still be kind of hack and should
be addressed with /branches refactoring.