Remove ADD_ISFINANCED_DEVICE_FLAG

ADD_ISFINANCED_DEVICE_FLAG defaults to true, and no one appears to be
overriding it via DeviceConfig.  Therefore, all the code that is
conditional on this flag being false is unused.  Remove it.

Bug: 259908270
Flag: EXEMPT removing already-launched flag
Test: atest Frameworks{,Mocking}ServicesTests:com.android.server.devicepolicy
Change-Id: I8c38a4a6163006b9788a002d5d11fb87603aa292