dirty_memory_manager: drop memory_hard_limit::shutdown()

It is empty.

dirty_memory_manager.hh

@@ -225,10 +225,6 @@ public:
     void add(region_group* child);
     void del(region_group* child);
 
-    future<> shutdown() {
-        return make_ready_future<>();
-    }
-
     template <region_group_or_memory_hard_limit RG>
     friend void do_update(RG* rg, RG*& top_relief, ssize_t delta);
 

replica/database.cc

@@ -1729,9 +1729,7 @@ future<> dirty_memory_manager::shutdown() {
     _db_shutdown_requested = true;
     _should_flush.signal();
     return std::move(_waiting_flush).then([this] {
-        return _virtual_region_group.shutdown().then([this] {
-            return _real_region_group.shutdown();
-        });
+        return _virtual_region_group.shutdown();
     });
 }