Remove Python < 3.7 timezone workaround in _rfc3339.py

Python 3.7+ supports %z with colons in timezone offsets. Since this
project now requires Python >= 3.9, the regex substitution to remove
colons from timezone offsets is no longer necessary and can be removed.

Co-authored-by: lahirumaramba <55609+lahirumaramba@users.noreply.github.com>

Author: google-labs-jules[bot]

firebase_admin/_rfc3339.py

@@ -68,12 +68,6 @@ def _parse_to_datetime(datestr):
     except ValueError:
         pass
 
-    # Note: %z parses timezone offsets, but requires the timezone offset *not*
-    # include a separating ':'. As of python 3.7, this was relaxed.
-    # TODO(rsgowman): Once python3.7 becomes our floor, we can drop the regex
-    # replacement.
-    datestr_modified = re.sub(r'(\d\d):(\d\d)$', r'\1\2', datestr_modified)
-
     try:
         return datetime.strptime(datestr_modified, '%Y-%m-%dT%H:%M:%S.%f%z')
     except ValueError: