wifi(implementation): Remove support for STA + STA in chip combination

Not planning to support STA + STA in Pixel 2018, so remove support for
it from the chip combination.

Also, fixed an error in existing unit tests (Need to mock
WifiLegacyHal.registerRadioModeChangeCallbackHandler)

Bug: 74079118
Test: Tested STA connection & AP bringup on taimen.
Test: ./hardware/interfaces/wifi/1.2/default/tests/runtests.sh
Change-Id: I996e63ecdda4d6a5a07a40d5f7ed1072ada37597