Revert "ci: add rebase-required mode to check-maintenance action" (#24179)

.github/MAINTAINER.md

@@ -150,18 +150,5 @@ When the CI is unhealthy (e.g., the scheduled pr-test on `main` is broken for co
 
 Maintenance mode ends when `pr-test.yml` is all green on `main` and the issue is closed.
 
-### Rebase-Required Mode
-When a major update lands on `main` and all open PRs must rebase before CI can run (without fully pausing CI), add a line of the form `MIN_BASE_SHA: <sha>` to the body of issue #21065. **The rebase check is enforced regardless of whether the issue is open or closed** — you do not need to enter full maintenance mode (open the issue) to use this gate; just editing the body to include the directive is enough. While the directive is present:
-- CI is allowed to run only for PRs whose branch already contains `<sha>` (GitHub compare API status `ahead` or `identical` — i.e., the PR has `<sha>` in its history).
-- PRs that are `behind` or `diverged` from `<sha>` are blocked with a "rebase required" error until they rebase onto the latest `main`.
-- The `bypass-maintenance` label still bypasses this check for CI-fix PRs.
-
-Notes:
-- Only the **first** `MIN_BASE_SHA:` line in the issue body is read.
-- The SHA must be 7-40 hex characters; malformed values are ignored (with a warning in the job summary).
-- Avoid pasting the directive inside a fenced code block in the issue body — the parser does not skip code fences and may match example snippets.
-
-Remove the directive from the issue body to lift the rebase requirement (closing the issue does NOT lift it on its own).
-
 ## Suspending Permissions
 If a Merge Oncall bypasses checks to merge a PR that breaks the `main` branch, merges a non-CI-fix PR during CI Maintenance Mode, or repeatedly breaks the CI due to various reasons, their privileges will be suspended for at least two days, depending on the severity of the incident.