Kernel: Remove MemoryManager::region_tree() accessor

Let's not have a way to grab at the RegionTree from outside of MM.

Kernel/Memory/MemoryManager.cpp

@@ -663,7 +663,7 @@ Region* MemoryManager::kernel_region_from_vaddr(VirtualAddress address)
     if (is_user_address(address))
         return nullptr;
 
-    return MM.region_tree().find_region_containing(address);
+    return MM.m_region_tree.find_region_containing(address);
 }
 
 Region* MemoryManager::find_user_region_from_vaddr_no_lock(AddressSpace& space, VirtualAddress vaddr)

Kernel/Memory/MemoryManager.h

@@ -248,8 +248,6 @@ public:
 
     IterationDecision for_each_physical_memory_range(Function<IterationDecision(PhysicalMemoryRange const&)>);
 
-    auto& region_tree() { return m_region_tree; }
-
 private:
     MemoryManager();
     ~MemoryManager();