Documentation: remove trivial tree

As has been discussed some time ago on ksumitt-discuss@ mailinglist,
the need for trivial tree diminished over time as all the tooling and
processess became much more mature and it's quite natural these days
for trivial patches to flow through subsystem trees anyway, so the
spin-off of a trivial tree doesn't make sense any more, and is not worth
the merge conflicts it might sometimes create.

So remove any mentions of it from kernel documentation for good.

Link: https://lore.kernel.org/all/nycvar.YFH.7.76.2104222334290.18270@cbobk.fhfr.pm/


Signed-off-by: Jiri Kosina <jkosina@suse.cz>