submitting-patches.rst: remove heading numbering

This follows similar changes throughout Documentation; these numbers
tend to get outdated and are not especially useful.

Signed-off-by: Drew DeVault <sir@cmpwn.com>
Link: https://lore.kernel.org/r/20200903160545.83185-2-sir@cmpwn.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>