Kernel: Remove unused list of user regions in MemoryManager

We were putting all the user Region objects on an intrusive list
but not using it for anything.

Kernel/Memory/MemoryManager.cpp

@@ -1067,8 +1067,6 @@ void MemoryManager::register_region(Region& region)
     ScopedSpinLock lock(s_mm_lock);
     if (region.is_kernel())
         m_kernel_regions.append(region);
-    else
-        m_user_regions.append(region);
 }
 
 void MemoryManager::unregister_region(Region& region)
@@ -1076,8 +1074,6 @@ void MemoryManager::unregister_region(Region& region)
     ScopedSpinLock lock(s_mm_lock);
     if (region.is_kernel())
         m_kernel_regions.remove(region);
-    else
-        m_user_regions.remove(region);
 }
 
 void MemoryManager::dump_kernel_regions()

Kernel/Memory/MemoryManager.h

@@ -285,7 +285,6 @@ private:
     PhysicalPageEntry* m_physical_page_entries { nullptr };
     size_t m_physical_page_entries_count { 0 };
 
-    Region::ListInMemoryManager m_user_regions;
     Region::ListInMemoryManager m_kernel_regions;
     Vector<UsedMemoryRange> m_used_memory_ranges;
     Vector<PhysicalMemoryRange> m_physical_memory_ranges;