net: x25: Remove unimplemented X.25-over-LLC code stubs

According to the X.25 documentation, there was a plan to implement
X.25-over-802.2-LLC. It never finished but left various code stubs in the
X.25 code. At this time it is unlikely that it would ever finish so it
may be better to remove those code stubs.

Also change the documentation to make it clear that this is not a ongoing
plan anymore. Change words like "will" to "could", "would", etc.

Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
Link: https://lore.kernel.org/r/20201209033346.83742-1-xie.he.0141@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

docs: networking: convert x25.txt to ReST

Not much to be done here:
- add SPDX header;
- add a document title;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>