contrib/subtree: remove "push" command from the "todo" file

Because the "push" command is already available, remove it from the
"todo" file.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/todo

@@ -12,8 +12,6 @@
 		exactly the right subtree structure, rather than using
 		subtree merge...)
 
-	add a 'push' subcommand to parallel 'pull'
-	
 	add a 'log' subcommand to see what's new in a subtree?
 
 	add to-submodule and from-submodule commands