platform: Drop timer warm init and exit hooks

Now that driver lifecycle is managed from within the SBI timer core,
platforms need only to initialize the driver once during cold init.
Remove the remaining platform hooks that are no longer used.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

lib/utils/timer/fdt_timer_mtimer.c

@@ -165,6 +165,4 @@ static const struct fdt_match timer_mtimer_match[] = {
 struct fdt_timer fdt_timer_mtimer = {
 	.match_table = timer_mtimer_match,
 	.cold_init = timer_mtimer_cold_init,
-	.warm_init = NULL,
-	.exit = NULL,
 };