Remove "I have added an entry to CHANGELOG.md" from the PR template

[dbeatty10]

Describe the "under the hood" change

As described in the release process instructions, we have been using the "Generate release notes" button in GitHub in place of manual changelog entries.

So we should remove this line from our PR template since it's not something users need to do. In fact, if they add one manually, we'll need to remove it.

Who will this benefit?

This will reduce the length of the PR checklist, which is easier to focus on (in alignment with the suggestions within the Checklist Manifesto).