Bug 1155283 - remove XULAPP_DEFINES from DIST_FILES_FLAGS; r=mshal

Nothing uses XULAPP_DEFINES anymore, let's just get rid of it.  There
are plenty more places to add in defines for DIST_FILES if one is so
inclined.

config/rules.mk

@@ -1268,7 +1268,7 @@ ifneq ($(DIST_FILES),)
 DIST_FILES_PATH := $(FINAL_TARGET)
 # We preprocess these, but they don't necessarily have preprocessor directives,
 # so tell them preprocessor to not complain about that.
-DIST_FILES_FLAGS := $(XULAPP_DEFINES) --silence-missing-directive-warnings
+DIST_FILES_FLAGS := --silence-missing-directive-warnings
 PP_TARGETS += DIST_FILES
 endif