Remove MAINDEXCLASSES

The last user of MAINDEXCLASSES was removed in
Ia88660550c3e57749b8ccb154e97c31aaf2fcf3a.

Test: m checkbuild
Change-Id: I35d9e7d9ff4ac2c07716db59c900b8c3f2cb352e
diff --git a/core/config.mk b/core/config.mk
index 0b317fb..bead8f6 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -490,11 +490,8 @@
 #
 ifeq (,$(TARGET_BUILD_USE_PREBUILT_SDKS))
   AAPT := $(HOST_OUT_EXECUTABLES)/aapt
-  MAINDEXCLASSES := $(HOST_OUT_EXECUTABLES)/mainDexClasses
-
 else # TARGET_BUILD_USE_PREBUILT_SDKS
   AAPT := $(prebuilt_sdk_tools_bin)/aapt
-  MAINDEXCLASSES := $(prebuilt_sdk_tools)/mainDexClasses
 endif # TARGET_BUILD_USE_PREBUILT_SDKS
 
 ifeq (,$(TARGET_BUILD_USE_PREBUILT_SDKS))