xtensa: drop platform_heartbeat

xtensa: drop platform_heartbeat

platform_heartbeat is called from the timer interrupt handler, but
there may be no periodic timer interrupts on xtensa, so the frequency of
platform_heartbeat calls may be unrelated to HZ. Drop the callback and
reimplement its only user with a timer.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>