Clean up pull request template (#1831)

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

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

.github/pull_request_template.md

@@ -1,11 +1,3 @@
----
-name: Spec clarification/not a proposal
-about: A change that's not a spec proposal, such as a clarification to the spec itself.
-title: ''
-labels: ''
-assignees: ''
-
----
 
 ### Pull Request Checklist
 

changelogs/internal/newsfragments/1831.clarification

@@ -0,0 +1 @@
+Clean up pull request template.