ci: do not constantly rebase PRs

It seems the 'rebase' action in Mergify causes each PR to be
automatically rebased once there is a change in the target branch. This
causes a very high load on the CI, which delays other PRs from getting
their test results.

Remove the 'rebase' action, as it does not work as intended. The
expectation was that '@mergifyio rebase' uses the configuration.

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

.mergify.yml

@@ -1,10 +1,5 @@
 ---
 pull_request_rules:
-  - name: rebase on request
-    conditions: []
-    actions:
-      rebase:
-        bot_account: ceph-csi-bot
   - name: remove outdated approvals
     conditions:
       - base=master