Style refactoring in the `linkcheck` builder (#11501)

- Omit a variable that is unused aside from a ``return`` statement on the subsequent line.
- Extract a variable for readability, and to reduce the complexity of a line.

sphinx/builders/linkcheck.py

@@ -441,7 +441,8 @@ class HyperlinkAvailabilityCheckWorker(Thread):
             return 'broken', error_message, 0
 
         # Success; clear rate limits for the origin
-        self.rate_limits.pop(urlsplit(req_url).netloc, None)
+        netloc = urlsplit(req_url).netloc
+        self.rate_limits.pop(netloc, None)
 
         if ((response_url.rstrip('/') == req_url.rstrip('/'))
                 or _allowed_redirect(req_url, response_url,
@@ -503,8 +504,7 @@ def _get_request_headers(
 
     for u in candidates:
         if u in request_headers:
-            headers = {**DEFAULT_REQUEST_HEADERS, **request_headers[u]}
-            return headers
+            return {**DEFAULT_REQUEST_HEADERS, **request_headers[u]}
     return {}