sound: Remove the since deleted dummy.c sound driver from the Makefile.

The file itself had been deleted, but it was still in the Makefile.

BUG=None
TEST=Built for pit.
BRANCH=None

Change-Id: I7a384943dff40354e0e6604300c82362ec9060e0
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://gerrit.chromium.org/gerrit/64880
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Tested-by: Gabe Black <gabeblack@chromium.org>
diff --git a/src/drivers/sound/Makefile.inc b/src/drivers/sound/Makefile.inc
index 5964e08..c227da2 100644
--- a/src/drivers/sound/Makefile.inc
+++ b/src/drivers/sound/Makefile.inc
@@ -17,7 +17,6 @@
 ## Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
 ##
 
-depthcharge-$(CONFIG_DRIVER_SOUND_DUMMY) += dummy.c
 depthcharge-$(CONFIG_DRIVER_SOUND_HDA) += hda_codec.c
 depthcharge-$(CONFIG_DRIVER_SOUND_I2S) += i2s.c
 depthcharge-$(CONFIG_DRIVER_SOUND_MAX98095) += max98095.c