[github] Remove assignees in CP requests (#137729)

b/307993619

FRoB will assign the RelEng oncall to these issues.

.github/ISSUE_TEMPLATE/7_cherry_pick.yml

@@ -2,9 +2,6 @@ name: Request a cherry-pick
 description: As a contributor, you would like to request that a feature be cherry-picked into a release.
 title: '[CP] <title>'
 labels: ['cp: review']
-assignees:
-  - itsjustkevin
-  - caseyhillers
 body:
   - type: input
     id: issue_link