audio: Remove unused var and unnecessary code

Remove update_mixer (as its unused) and change function signatures
wherever used.
Remove code chunk to set BT sample rate before enabling device as
thats part of the mixer path file now.

Change-Id: I5eb3bce5183acf505596be2b0ec93cdbd728bdaa
CRs-Fixed: 581453
diff --git a/hal/voice.c b/hal/voice.c
index ac067a3..62d01db 100644
--- a/hal/voice.c
+++ b/hal/voice.c
@@ -88,11 +88,11 @@
     }
 
     /* 2. Get and set stream specific mixer controls */
-    disable_audio_route(adev, uc_info, true);
+    disable_audio_route(adev, uc_info);
 
     /* 3. Disable the rx and tx devices */
-    disable_snd_device(adev, uc_info->out_snd_device, false);
-    disable_snd_device(adev, uc_info->in_snd_device, true);
+    disable_snd_device(adev, uc_info->out_snd_device);
+    disable_snd_device(adev, uc_info->in_snd_device);
 
     list_remove(&uc_info->list);
     free(uc_info);