Revert "Update PR template to add clear link requirement. (#476)"

This reverts commit 4d96d72.

Author: Wei Weng

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,7 +13,6 @@ Fixes #
 
 I have:
 
-- [ ] Associated this change with a known KubeFleet Issue (Bug, Feature, etc).
 - [ ] Run `make reviewable` to ensure this PR is ready for review.
 
 ### How has this code been tested