hal: remove HAL access to channel number mixer control

Channel number is hardware-specific and defined by mixer paths.
With access to channel mixer control, HAL would overwrite the
correct channel number. Disallowing HAL makes mixer paths value
take place.

Change-Id: I01de2dd5d3cb85b1b3cf8949feb1d8914ee4a298
diff --git a/hal/msm8974/platform.c b/hal/msm8974/platform.c
index 2ac3852..948703f 100644
--- a/hal/msm8974/platform.c
+++ b/hal/msm8974/platform.c
@@ -2544,8 +2544,6 @@
         strdup("SLIM_0_RX Format");
     my_data->current_backend_cfg[DEFAULT_CODEC_BACKEND].samplerate_mixer_ctl =
         strdup("SLIM_0_RX SampleRate");
-    my_data->current_backend_cfg[DEFAULT_CODEC_BACKEND].channels_mixer_ctl =
-        strdup("SLIM_0_RX Channels");
 
     my_data->current_backend_cfg[DSD_NATIVE_BACKEND].bitwidth_mixer_ctl =
         strdup("SLIM_2_RX Format");