tests: remove bundle2 activation from test-push-warn.t

This is an old config that predate bundle2 on by default. This should have been
remove after Mercurail 3.6 got released.

tests/test-push-warn.t

@@ -1,9 +1,3 b''
-  $ cat << EOF >> $HGRCPATH
-  > [experimental]
-  > # drop me once bundle2 is the default,
-  > # added to get test change early.
-  > bundle2-exp = True
-  > EOF
   $ hg init a
   $ cd a
   $ echo foo > t1