realtek: Remove _machine_restart and _machine_halt

By dropping _machine_restart, users can provide more reliable or
device-specific restart modes.

_machine_halt was already removed in commit f4b687d1 ("realtek: use
kernel defined halt"), but quietly reintroduced in commit 8faffa00
("realtek: add support for the RTL9300 timer"). Let's remove it again.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Tested-by: Stijn Segers <foss@volatilesystems.org>
Tested-by: Paul Fertser <fercerpav@gmail.com>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>