Remove USE_XML_AUDIO_POLICY_CONF build flag

This flag is no more used by the build files and should
be removed.

Bug: 132435217
Test: make
Change-Id: I5486e13ee5b746a7ae65a184277d5caa6d2d596f
diff --git a/device-common.mk b/device-common.mk
index 525ae8d..df15455 100644
--- a/device-common.mk
+++ b/device-common.mk
@@ -221,7 +221,6 @@
         $(LOCAL_PATH)/p2p_supplicant_overlay.conf:$(TARGET_COPY_OUT_VENDOR)/etc/wifi/p2p_supplicant_overlay.conf
 
 # audio policy configuration
-USE_XML_AUDIO_POLICY_CONF := 1
 PRODUCT_COPY_FILES += \
     device/linaro/hikey/audio/audio_policy_configuration.xml:$(TARGET_COPY_OUT_VENDOR)/etc/audio_policy_configuration.xml \
     frameworks/av/services/audiopolicy/config/a2dp_audio_policy_configuration.xml:$(TARGET_COPY_OUT_VENDOR)/etc/a2dp_audio_policy_configuration.xml \