memory: eliminate global MemoryListeners

There is none, so just drop the code.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/include/exec/memory.h b/include/exec/memory.h
index 10d7eac..39f3410 100644
--- a/include/exec/memory.h
+++ b/include/exec/memory.h
@@ -255,7 +255,7 @@
                                hwaddr addr, hwaddr len);
     /* Lower = earlier (during add), later (during del) */
     unsigned priority;
-    AddressSpace *address_space_filter;
+    AddressSpace *address_space;
     QTAILQ_ENTRY(MemoryListener) link;
 };