Undocument deprecated alias 'push.default=tracking'

It's been deprecated since 53c4031 (Johan Herland, Wed Feb 16 2011,
push.default: Rename 'tracking' to 'upstream'), so it's OK to remove it
from documentation (even though it's still supported) to make the
explanations more readable.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1690,7 +1690,6 @@ push.default::
   With this, `git push` will update the same remote ref as the one which
   is merged by `git pull`, making `push` and `pull` symmetrical.
   See "branch.<name>.merge" for how to configure the upstream branch.
-* `tracking` - deprecated synonym for `upstream`.
 * `current` - push the current branch to a branch of the same name.
   +
   The `current` and `upstream` modes are for those who want to