Remove doc/old/nommu-notes.txt

UCLINUX is dead and it's support has been removed from LTP.

Although Documentation/admin-guide/mm/nommu-mmap.rst still exists in
kernel tree, there is nobody really using it.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Cyril Hrubis <chrubis@suse.cz>