Clean up ravenwood mockito support.

- Remove the "mockito_ravenwood" module, which is based on mockito
ver 2, now that we use the robo version of mockito (ver 4).

- Remove the use of the deprecated mockito API, which is not
supported by mockito ver 4.

- Add a test for mocking final class.

- Remove the stale MD file about mockito.

Bug: 310268946
Test: ./run-ravenwood-tests.sh
Change-Id: I236661df47f40e0b05805156908e58ff0d2ae535