a2dp: Remove disabling of rate scaling for certain car headunits

It seems like it was originally made for Porsche PCM headunits but the device
detection wasn't specific enough. On my 2012 Audi A6 with a MMI touch
headunit it caused a lot of cracks and pops in the audio. Removing it brought
the audio quality back to the same level it was at in Android 4.1.

Change-Id: I009f25f9fddcf6c08c7e063797bd282ef89dc355