MIPS: ath79: drop _machine_restart again

Commit 81424d0ad0d4 ("MIPS: ath79: Use the reset controller to restart
OF machines") removed setup of _machine_restart on OF machines to use
reset handler in reset controller driver.
While removing remnants of non-OF machines in commit 3a77e0d75eed
("MIPS: ath79: drop machfiles"), this was introduced again, making it
impossible to use additional restart handlers registered through device
tree. Drop setting _machine_restart altogether, and ath79_restart
function, which is no longer used after this.

Fixes: 3a77e0d75eed ("MIPS: ath79: drop machfiles")
Cc: John Crispin <john@phrozen.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>