dma-buf/sw-sync: Remove unused debug code

sync_file_debug_add() and sync_file_debug_remove() have been unused
since 2016's
commit d4cab38e ("staging/android: prepare sync_file for de-staging")

Remove them.

Since sync_file_debug_add was the only thing to add to
sync_file_list_head, the code that dumps it in part of
sync_info_debugfs_show can be removed, and the declaration of
the list and it's associated lock can be removed.
(The 'fences:\n...' marker in that debugfs file is left in
so as not to change the output)

That leaves the sync_print_sync_file() helper unused, and
is thus removed.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://lore.kernel.org/r/20250505233838.105668-1-linux@treblig.org