qemu-timer: drop outdated signal safety comments

host_alarm_handler() is invoked from the signal processing thread
(currently the iothread).  Previously we did processing in a real signal
handler with signalfd and therefore needed signal-safe timer code.

Today host_alarm_handler() just marks the alarm timer as expired/pending
and notifies the main loop using qemu_notify_event().

Therefore these outdated comments about signal safety can be dropped.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
