ipm: remove ipm_cavs_idc driver

The ipm_cavs_idc driver was used with the old intel_s1000
board which has been removed. On the audio DSP side,
the IDC under CAVS is being handled by SoC layer code.
Now the ipm_cavs_idc is not needed anymore for anything.
So remove it.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>