CRAS: Remove doc about BluetoothBatteryChanged signal

The BluetoothBatteryChanged is no longer needed after the migration to
BlueZ Battery Provider API. The signal itself has been removed in
CL:2521465.

BUG=b:172361090
TEST=None

Change-Id: I448587b7b3c154914444798154ecde09a9e49e60
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/2593236
Reviewed-by: Hsinyu Chao <hychao@chromium.org>
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Tested-by: Sonny Sasaka <sonnysasaka@chromium.org>
1 file changed