checklist: remove gpg key requirement

Requiring that all commits are signed is not strictly necessary. We
should still encourage it, and perhaps inspect commits more closely
before merging them if unsigned, but this is a barrier to entry for the
Hyades project that we do not need.

.github/pull_request_template.md

@@ -10,7 +10,6 @@ Checklist:
  - [ ] The first line of each commit is of the form "[component]: do something"
  - [ ] I have written a complete, multi-line commit message for each commit.
  - [ ] I have formatted any Go code that I have changed with gofmt.
- - [ ] I have signed each commit with my GPG key.
  - [ ] I have written or updated appropriate documentation to cover this change.
  - [ ] I have confirmed that this change is covered by at least one appropriate test run by CI.
  - [ ] If my change includes new or modified functionality, I have tested that the changes work as expected.