Remove kind/design from PR template

- Remove `kind/design` from PR template in lieu of kubernetes/community#5641
- Removal of `kind/design` from `k/k` is made possible by kubernetes/test-infra#23011

Signed-off-by: Madhav Jivrajani <[email protected]>

Author: MadhavJivrajani

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,7 +16,6 @@ Add one of the following kinds:
 /kind cleanup
 /kind documentation
 /kind feature
-/kind design
 
 Optionally add one or more of the following kinds if applicable:
 /kind api-change