Don't set DISABLE_DEXPREOPT in lunch.

Because in core/main.mk WITH_DEXPREOPT is set to true only if the build OS is linux.

Change-Id: I287ab61312667f6100371d07010bcab5cfcc7e17
diff --git a/envsetup.sh b/envsetup.sh
index e85aab6..a8f2d4d 100644
--- a/envsetup.sh
+++ b/envsetup.sh
@@ -150,9 +150,6 @@
     setpaths
     set_sequence_number
 
-    # Don't try to do preoptimization until it works better on OSX.
-    export DISABLE_DEXPREOPT=true
-
     export ANDROID_BUILD_TOP=$(gettop)
 }