submitting-patches.rst: presume git will be used

submitting-patches.rst: presume git will be used

Git is fairly ubiquitous these days, and the additional information in
this documentation for preparing patches without it is not especially
relevant anymore and may serve to confuse new contributors.

The git request-pull comments were also removed, given that it is not a
tool well-suited to novice contributors, nor do maintainers especially
appreciate receiving unexpected request-pulls from new contributors.

Signed-off-by: Drew DeVault <sir@cmpwn.com>
Link: https://lore.kernel.org/r/20200903160545.83185-5-sir@cmpwn.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>