extensions: obsolete and remove parentrevspec extension

As of 305c97670d7a, revsets implements the ^ and ~ operators that this
extension provides, so it's no longer necessary.

mercurial/extensions.py

@@ -11,7 +11,7 b' from i18n import _, gettext'
 
 _extensions = {}
 _order = []
-_ignore = ['hbisect', 'bookmarks']
+_ignore = ['hbisect', 'bookmarks', 'parentrevspec']
 
 def extensions():
     for name in _order: