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: #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>

dev/merge_arrow_pr.py

@@ -418,11 +418,23 @@ def merge_pr(self, number, commit_title, commit_message):
         }
         response = requests.put(url, headers=self.headers, json=payload)
         result = response.json()
-        if response.status_code != 200 and 'merged' not in result:
+        if response.status_code == 200 and 'merged' in result:
+            self.clear_pr_state_labels(number)
+        else:
             result['merged'] = False
             result['message'] += f': {url}'
         return result
 
+    def clear_pr_state_labels(self, number):
+        url = f'{self.github_api}/issues/{number}/labels'
+        response = requests.get(url, headers=self.headers)
+        labels = response.json()
+        for label in labels:
+            # All PR workflow state labes starts with "awaiting"
+            if label['name'].startswith('awaiting'):
+                label_url = f"{url}/{label['name']}"
+                requests.delete(label_url, headers=self.headers)
+
 
 class CommandInput(object):
     """