[minor-refactor] Remove unused title parameter on merge request creation helper.

Every new merge request path will pass through the `new` action: https://github.com/gitlabhq/gitlabhq/blob/cb0d63b9e3fa464ae8c0cd9cf7bdb8d080a7e02f/app/controllers/projects/merge_requests_controller.rb#L60, which in turn uses `MergeRequests::BuildService`, which fixes the title to a humanized branch name no matter what `:title` parameter you pass: https://github.com/gitlabhq/gitlabhq/blob/cb0d63b9e3fa464ae8c0cd9cf7bdb8d080a7e02f/app/services/merge_requests/build_service.rb#L20