remove unused page_header and page_subhead content blocks

closes CNVS-14803

test plan:
- make sure the headers are unchanged on the following pages:
  - /accounts/1/account_authorization_configs
  - /accounts/1/sis_import
  - /courses/1/content_exports
  - /courses/1/confirm_action?event=delete
  - /accounts/1/terms
  - /accounts/1/sub_accounts
  - /accounts/1/users/1/delete
  - /users/1/admin_merge

Change-Id: I4563e243cb733e2826d52b408991032da3767efa
Reviewed-on: https://gerrit.instructure.com/39373
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>