Fix mac build

As reported here:
https://groups.google.com/d/msg/android-building/lGawwN0LQP0/KrY194rCCAAJ

WITH_DEXPREOPT already defaults to true on Linux, and is not currently
supported on Mac.

Test: m  (on mac)
Change-Id: Ic3e93fac5a20cce643436406a8e93ff4e28b4f5f
diff --git a/BoardConfigCommon.mk b/BoardConfigCommon.mk
index eb64ac6..3ab2410 100644
--- a/BoardConfigCommon.mk
+++ b/BoardConfigCommon.mk
@@ -11,7 +11,6 @@
 
 TARGET_USES_64_BIT_BINDER := true
 
-WITH_DEXPREOPT ?= true
 USE_OPENGL_RENDERER := true
 
 # BT configs