fix: remove required reviewer from pr template (#145)

Remove `@edx/arch-review` from the list of required reviewers.

I added the PR template as part of adding a changelog 2 years ago,
and I don't see any reason to also have added a new review requirement,
namely `@edx/arch-review`. It is possible this was just copy/pasted.

Most contributors are already accustomed to asking owners for review
when and if there is any significant change that requires review. This
change simplifies the process for simple changes by not implying that
arch-review (which is not actually the owner anyway) needs to review.

.github/PULL_REQUEST_TEMPLATE.md

@@ -14,7 +14,6 @@ Describe in a couple of sentences what this PR adds
 * **Author concerns:** List any concerns about this PR
 
 **Reviewers:**
-- [ ] @edx/arch-review
 - [ ] tag reviewer
 
 **Merge checklist:**