Remove obsolete code in BluetoothEventMangaer

1. Remove readPairedDevices() because we already did it in constructor.
There is no need to call it each time there is a device miss.
2. Remove dispatchDeviceAdd() since it is already been called in
addDevice().

Bug: 111803044
Test: test still pass
Change-Id: Ib69daf47b1227d31bf1e6ac17a7e0505046adf17