Remove the redundant DISABLE_DEXPREOPT

To disable dexpreopt, use just:
WITH_DEXPREOPT=false

Change-Id: I5804f3774f8ec50eb16c5336776dbce0c28b16d9
diff --git a/core/main.mk b/core/main.mk
index 2399d2e..3c4fd94 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -343,7 +343,7 @@
   # Turn on Dalvik preoptimization for user builds, but only if not
   # explicitly disabled and the build is running on Linux (since host
   # Dalvik isn't built for non-Linux hosts).
-  ifneq (true,$(DISABLE_DEXPREOPT))
+  ifeq (,$(WITH_DEXPREOPT))
     ifeq ($(user_variant),user)
       ifeq ($(HOST_OS),linux)
         WITH_DEXPREOPT := true