arrow

GH-36243: [Dev] Remove PR workflow label as part of merge (#36244)

### Rationale for this change

Those labels are unnecessary once they are merged. There was a conversation on Zulip about removing them in the past.

### What changes are included in this PR?

Once we merge a PR we remove labels that starts with the PR workflow prefix `awaiting`.

### Are these changes tested?

I have tested the code against an old testing PR I have here: https://github.com/apache/arrow/pull/35323
The label was removed successfully.

### Are there any user-facing changes?

No
* Closes: #36243

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>