docs: process: remove outdated submitting-drivers.rst

Commit 31b24bee ("docs: add a warning to submitting-drivers.rst")
in October 2016 already warns "This (...) should maybe just be deleted,
but I'm not quite ready to do that yet".

Maybe, six years ago, we were not ready but let us remove old content
for the better now and structure and maintain less content in the kernel
documentation with a better result.

Drop this already outdated document and adjust all textual references.

Here is an argument why deleting the content will not remove any useful
information to the existing kernel documentation, individually broken down
for each section.

Section "Allocating Device Numbers" refers to https://www.lanana.org/, and
then refers to Documentation/admin-guide/devices.rst.

However, the devices.rst clearly states:

  "The version of this document at lanana.org is no longer maintained."

Everything needed for submitting drivers is already stated in devices.rst
and the reference to https://www.lanana.org/ ...