hgweb: delete a local variable instead of setting to `None`

The previous code was flagged by PyCharm as an unused variable assignment.

Differential Revision: https://phab.mercurial-scm.org/D7761

mercurial/hgweb/wsgicgi.py

@@ -77,7 +77,7 b' def launch(application):'
                     # Re-raise original exception if headers sent
                     raise exc_info[0](exc_info[1], exc_info[2])
             finally:
-                exc_info = None  # avoid dangling circular ref
+                del exc_info  # avoid dangling circular ref
         elif headers_set:
             raise AssertionError(b"Headers already set!")