Don't build userdata.img for tests build.

Change-Id: I2982223919267f7b98f47b002c435e443674729a
diff --git a/core/Makefile b/core/Makefile
index a81c981..c7e5406 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1234,13 +1234,11 @@
 
 # Target needed by tests build
 .PHONY: tests-build-target
-tests-build-target: $(BUILT_TESTS_ZIP_PACKAGE) \
-                    $(BUILT_USERDATAIMAGE_TARGET)
+tests-build-target: $(BUILT_TESTS_ZIP_PACKAGE)
 
 ifneq (,$(filter $(MAKECMDGOALS),tests-build-target))
   $(call dist-for-goals, tests-build-target, \
-          $(BUILT_TESTS_ZIP_PACKAGE) \
-          $(BUILT_USERDATAIMAGE_TARGET))
+          $(BUILT_TESTS_ZIP_PACKAGE))
 endif
 
 # -----------------------------------------------------------------