Bug 1420092 - Don't always enable mozjemalloc when building the js engine. r=njn

The target.kernel-based tests should be enough.

--HG--
extra : rebase_source : 4af988ac952f3a65c638bf7bb308a8ddf4798bcf

build/moz.configure/memory.configure

@@ -19,9 +19,6 @@ def jemalloc(value, target, build_project, c_compiler):
     if value.origin != 'default':
         return bool(value) or None
 
-    if build_project == 'js':
-        return True
-
     if target.kernel in ('Darwin', 'Linux'):
         return True