timer: Remove reset notifiers

Remove the reset notifer from the core qemu-timer code.
The only user was mc146818 and we've just remove it's use.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190724115823.4199-3-dgilbert@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>