exthelper: drop a compatibility shim with '^command' syntax

exthelper: drop a compatibility shim with '^command' syntax

This syntax was dropped in fa88170c10bb.

--- a/mercurial/exthelper.py	Mon Dec 24 14:04:16 2018 -0500
+++ b/mercurial/exthelper.py	Thu Dec 27 00:10:40 2018 -0500
@@ -37,15 +37,6 @@
         self._duckpunchers = []
         self.cmdtable = {}
         self.command = registrar.command(self.cmdtable)
-        if '^init' in commands.table:
-            olddoregister = self.command._doregister
-
-            def _newdoregister(self, name, *args, **kwargs):
-                if kwargs.pop('helpbasic', False):
-                    name = '^' + name
-                return olddoregister(self, name, *args, **kwargs)
-            self.command._doregister = _newdoregister
-
         self.configtable = {}
         self.configitem = registrar.configitem(self.configtable)