relay: remove unused relay_late_setup_files

The last use of relay_late_setup_files() was removed in 2018 by commit
2b477330 ("drm/i915/guc: Merge log relay file and channel creation")

Remove it and the helper it used.

relay_late_setup_files() was used for eventually registering 'buffer only'
channels.  With it gone, delete the docs that explain how to do that. 
Which suggests it should be possible to lose the 'has_base_filename'
flags.

(Are there any other uses??)

Link: https://lkml.kernel.org/r/20250418234932.490863-1-linux@treblig.org


Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andriy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>