greybus: audio: Remove I2S Bridged-PHY Protcol based audio driver

The Greybus I2S Bridged-PHY Protocol is now deprecated so remove
the audio driver that is based on it.

CC: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>