qemu-trusty: dont build system-qemu.img

It is not needed.

Test:
lunch qemu_trusty_arm64-userdebug
make -j

Change-Id: I4c309f7f30c3aedba7754d20442eefd75bd990eb
diff --git a/BoardConfig.mk b/BoardConfig.mk
index 6fcb5c4..f2c2734 100644
--- a/BoardConfig.mk
+++ b/BoardConfig.mk
@@ -33,7 +33,6 @@
 
 TARGET_USES_64_BIT_BINDER := true
 
-BUILD_QEMU_IMAGES := true
 
 TARGET_USERIMAGES_USE_EXT4 := true
 BOARD_SYSTEMIMAGE_PARTITION_SIZE := 268435456 # 256M