Remove missing path causing warning

This is causing a warning when we read Android.mk files:

find: ‘tools/tradefederation/contrib/prod-tests/res/config’: No such file or directory

It will soon be an error.

Test: diff out/build-marlin.ninja before & after
Change-Id: Ib2eb43216cd8b362959b6e829f3e8f854dbf9ac8
diff --git a/Android.mk b/Android.mk
index ffbc110..8d2a653 100644
--- a/Android.mk
+++ b/Android.mk
@@ -14,7 +14,7 @@
 
 LOCAL_PATH := $(call my-dir)
 COMPATIBILITY.tradefed_tests_dir := \
-  $(COMPATIBILITY.tradefed_tests_dir) $(LOCAL_PATH)/res/config $(LOCAL_PATH)/prod-tests/res/config
+  $(COMPATIBILITY.tradefed_tests_dir) $(LOCAL_PATH)/res/config
 
 include $(CLEAR_VARS)