Clean up pull request template

[richvdh]

As far as I can tell, these header lines only encourage people to create badly-formatted PRs.

Also we only have one template so let's give it the default name.

Preview: https://pr1831--matrix-spec-previews.netlify.app

[erikjohnston]

Do you have an example of what this looks like? When I try and make a PR currently it seems to just give an empty box :/

[richvdh]

The thing I'm trying to fix is stuff like #1830.

[erikjohnston]

I mean sure why not.

Though I don't fully understand why I don't get PR templates :(

[tulir]

When I try and make a PR currently it seems to just give an empty box :/

I think that's because github's web UI doesn't support having multiple (named) PR templates, so when there's no default, it just doesn't show a template at all.

GitHub CLI does support those named templates, but it doesn't support the metadata section at the beginning

[sumnerevans]

I wasn't really sure what to put in that section, but it just keeps showing up whenever I do gh pr create which is the main way that I create PRs. It didn't really make sense to just copy the title and description that I already had written, so I just left it.

(I thought that it might be some sort of automation or accounting that was needed by some sort of process for the repo, so I just left it.)