hal: Clean up from merge

* Remove 8974-specific code where possible
* Don't distinguish based on PLATFORM_8960, everything is PLATFORM_8960
  if using the A-family HAL.
* Remove USECASE_AUDIO_RECORD_FM_VIRTUAL for 8960 platform, as it's not
  a valid codepath here.

Change-Id: I1cef978b27d7ea87fa3fa7af1e121545dfda1505