Remove clutter from PR template (#96)

The current template is very noisy when looking at `git log` (because it
shows the HTML comments) and a lot of it didn't apply to this project.

By submitting this pull request, I confirm that my contribution is made
under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,13 +1 @@
-Fixes #
-
-<!-- Is this a user-visible change?  Remember to update RELEASE_NOTES.md -->
-
-<!-- Is this a bug fix?  Remember to include a test in Test/git-issues/ -->
-
-<!-- Is this a bug fix for an issue introduced in the latest release?  Mention this in the PR details and ensure a patch release is considered -->
-
-<!-- Does this PR need tests?  Add them to `Test/` or to `Source/*.Test/…` and run them with `dotnet test` -->
-
-<!-- Are you moving a large amount of code? Read CONTRIBUTING.md to learn how to do that while maintaining git history -->
-
-<small>By submitting this pull request, I confirm that my contribution is made under the terms of the [MIT license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>
+By submitting this pull request, I confirm that my contribution is made under the terms of the [MIT license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).