Remove the freelist from memarea.c

This is in accordance with our usual policy against freelists,
now that working allocators are everywhere.

It should also make memarea.c's coverage higher.

I also doubt that this code ever helped performance.

changes/remove_memarea_freelist

@@ -0,0 +1,4 @@
+  o Removed code:
+    - We no longer maintain an internal freelist in memarea.c. Allocators
+      should be good enough to make this code unnecessary, and it's doubtful
+      that it ever had any performance benefit.