ci: drop ok-to-test label when a PR is rebased

When reviews are dropped, tests should most often not automatically
start again.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

.mergify.yml

@@ -59,6 +59,9 @@ pull_request_rules:
       dismiss_reviews:
         approved: true
         changes_requested: false
+      label:
+        remove:
+          - ok-to-test
 
   - name: ask to resolve conflict
     conditions: