obsolete: remove last instance of _enabled

The _enabled bool has been replaced by obsolete.isenabled(...). This removes the
last instance of it so I can remove the _enabled flag entirely shortly.

mercurial/cmdutil.py

@@ -2436,7 +2436,7 b' def commit(ui, repo, commitfunc, pats, o'
 def amend(ui, repo, commitfunc, old, extra, pats, opts):
     # amend will reuse the existing user if not specified, but the obsolete
     # marker creation requires that the current user's name is specified.
-    if obsolete._enabled:
+    if obsolete.isenabled(repo, obsolete.createmarkersopt):
         ui.username() # raise exception if username not set
 
     ui.note(_('amending changeset %s\n') % old)