.github: Remove the automatic rebase action (#2620)

Remove the `rebase` workflow that's responsible for automatically
rebasing open PRs. The peter-evans/rebase action requires additional
permissions than the default token we provide to actions at runtime, and
appears to only have support for failing fast when it encounters an
error rebasing an open PR, which isn't ideal for this repository.

Signed-off-by: timflannagan <[email protected]>

Author: timflannagan