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: Icbd2359da659290ce4fba1a11ba6da76f23679da
diff --git a/target/board/BoardConfigMainlineCommon.mk b/target/board/BoardConfigMainlineCommon.mk
index be7c804..64109e0 100644
--- a/target/board/BoardConfigMainlineCommon.mk
+++ b/target/board/BoardConfigMainlineCommon.mk
@@ -24,9 +24,6 @@
 # stripped from APKs.
 BOARD_USES_SYSTEM_OTHER_ODEX := true
 
-# Audio: must using XML format for Treblized devices
-USE_XML_AUDIO_POLICY_CONF := 1
-
 # Bluetooth defines
 # TODO(b/123695868): Remove the need for this
 BOARD_BLUETOOTH_BDROID_BUILDCFG_INCLUDE_DIR := build/make/target/board/mainline_arm64/bluetooth