docs: remove Documentation/dontdiff

docs: remove Documentation/dontdiff

The dontdiff file is a relic from the pre-Git era that has little use now.
It has entries (parse.c, for example) that will mask real changes to kernel
source files.  There are all kinds of entries for files we do not create
anymore.  Rather than try to fix it up, simply remove it.

Update the kernel documentation (and translations) to remove references to
this file.  There is an ancient Japanese translation of SubmittingPatches
that I am unable to update; that really needs a thorough redo.

Message-ID: <87y12m1zk4.fsf@trenco.lwn.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>