yaffs: Remove private list implementation

U-Boot already has a list implementation, and files which include both
that and the yaffs implementation will get errors:

In file included from ydirectenv.h:80:0,
                 from yportenv.h:81,
                 from yaffs_guts.h:19,
                 from yaffs_allocator.h:19,
                 from yaffs_allocator.c:14:
yaffs_list.h:32:8: error: redefinition of ‘struct list_head’
 struct list_head {
        ^

Remove the yaffs implementation.

Signed-off-by: Simon Glass <sjg@chromium.org>