stage: remove unused, unreferenced builtin-alias

11920d2 (Add a built-in alias for 'stage' to the 'add' command,
2008-12-01) added a the 'git stage' command which simply calls
cmd_add().  Since then, no references to it have been made anywhere on
the internet; there is no evidence that anyone even knows about its
existence.  It is a long-lost forgotten command that only serves the
purpose of preventing the user from having a custom alias.stage to do
something useful.  Remove it.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>