Don't delete `prose-patch` branch after submitting patch, fixes #257

When a non-collaborator user clicks commit, prose silently forks the repo, creates a branch, submits a pull request, and deletes the branch. Deleting the branch triggers GitHub to close the pull request, making a subsequent merge impossible.

This patch removes the logic from `_includes/model.js` that deletes the branch following a pull request. However, further refinement of the patching workflow may be required (e.g., to create a new branch for each pull request to avoid smushing commits together).

Tested successfully using https://github.com/benbalter/test-repo-ignore-me/pull/5.