Remove "Label when approved" workflow (#24704)

The labelling workflow has proven to be far less useful than we
thought and some of the recent changes in selective checks made
it largely obsolete. The committers can still add "full tests needed"
label when they think it is needed and there is no need to label
the PRs automatically for that (or any other reason).

For quite a while this workflow is basically a useless noise.

dev/airflow-github

@@ -126,7 +126,6 @@ def is_core_commit(files: List[str]) -> bool:
         "CONTRIBUTORS_QUICK_START.rst",
         "IMAGES.rst",
         "LOCAL_VIRTUALENV.rst",
-        "PULL_REQUEST_WORKFLOW.rst",
         "INTHEWILD.md",
         "INSTALL",
         "README.md",