Remove checklist from the PR template (#9702)

## Problem
Once we enable the merge queue for the `main` branch, it won't be
possible to adjust the commit message right after pressing the "Squash
and merge" button and the PR title + description will be used as is.

To avoid extra noise in the commits in the `main` with the checklist
leftovers, I propose removing the checklist from the PR template and
keeping only the Problem / Summary of changes.

## Summary of changes
- Remove the checklist from the PR template

.github/pull_request_template.md

@@ -1,14 +1,3 @@
 ## Problem
 
 ## Summary of changes
-
-## Checklist before requesting a review
-
-- [ ] I have performed a self-review of my code.
-- [ ] If it is a core feature, I have added thorough tests.
-- [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
-- [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.
-
-## Checklist before merging
-
-- [ ] Do not forget to reformat commit message to not include the above checklist