Kernel: Use default constructors/destructors

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#cother-other-default-operation-rules

"The compiler is more likely to get the default semantics right and
you cannot implement these functions better than the compiler."

Kernel/Memory/PhysicalRegion.cpp

@@ -27,9 +27,7 @@ static constexpr u32 next_power_of_two(u32 value)
     return value;
 }
 
-PhysicalRegion::~PhysicalRegion()
-{
-}
+PhysicalRegion::~PhysicalRegion() = default;
 
 PhysicalRegion::PhysicalRegion(PhysicalAddress lower, PhysicalAddress upper)
     : m_lower(lower)