Back out 21284a1b338f (bug 870366) for breaking Android nightlies, a=backout

CLOBBER

@@ -22,4 +22,4 @@
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Bug 870366 - Blacklisting PREF_JS_EXPORTS in Makefile.ins (because of 852814)
+Bug 1109248 - This needed a CLOBBER on Windows and OSX.

config/baseconfig.mk

@@ -90,7 +90,6 @@ _MOZBUILD_EXTERNAL_VARIABLES := \
   NO_DIST_INSTALL \
   OS_LIBS \
   PARALLEL_DIRS \
-  PREF_JS_EXPORTS \
   PROGRAM \
   PYTHON_UNIT_TESTS \
   RESOURCE_FILES \