LibJS: Use default instead of an empty constructor/destructor

Default implementations allow for more optimizations.
See: https://pvs-studio.com/en/docs/warnings/v832/

Userland/Libraries/LibJS/Heap/CellAllocator.cpp

@@ -17,10 +17,6 @@ CellAllocator::CellAllocator(size_t cell_size)
 {
 }
 
-CellAllocator::~CellAllocator()
-{
-}
-
 Cell* CellAllocator::allocate_cell(Heap& heap)
 {
     if (m_usable_blocks.is_empty()) {

Userland/Libraries/LibJS/Heap/CellAllocator.h

@@ -17,7 +17,7 @@ namespace JS {
 class CellAllocator {
 public:
     explicit CellAllocator(size_t cell_size);
-    ~CellAllocator();
+    ~CellAllocator() = default;
 
     size_t cell_size() const { return m_cell_size; }
 

Userland/Libraries/LibJS/Heap/Handle.h

@@ -41,7 +41,7 @@ public:
 template<class T>
 class Handle {
 public:
-    Handle() { }
+    Handle() = default;
 
     static Handle create(T* cell)
     {