Kernel: Remove unnecessary counting of VMObject-attached Regions

VMObject already has an IntrusiveList of all the Regions that map it.
We were keeping a counter in addition to this, and only using it in
a single place to avoid iterating over the list in case it only had
1 entry.

Simplify VMObject by removing this counter and always iterating the
list even if there's only 1 entry. :^)

Kernel/VM/VMObject.cpp

@@ -31,7 +31,7 @@ VMObject::~VMObject()
     }
 
     MM.unregister_vmobject(*this);
-    VERIFY(m_regions_count.load(AK::MemoryOrder::memory_order_relaxed) == 0);
+    VERIFY(m_regions.is_empty());
 }
 
 }