Libraries: Use default constructors/destructors in LibJS

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."

Userland/Libraries/LibJS/Heap/BlockAllocator.cpp

@@ -17,10 +17,6 @@
 
 namespace JS {
 
-BlockAllocator::BlockAllocator()
-{
-}
-
 BlockAllocator::~BlockAllocator()
 {
     for (auto* block : m_blocks) {

Userland/Libraries/LibJS/Heap/BlockAllocator.h

@@ -13,7 +13,7 @@ namespace JS {
 
 class BlockAllocator {
 public:
-    BlockAllocator();
+    BlockAllocator() = default;
     ~BlockAllocator();
 
     void* allocate_block(char const* name);

Userland/Libraries/LibJS/Heap/Cell.h

@@ -19,7 +19,7 @@ class Cell {
 
 public:
     virtual void initialize(GlobalObject&) { }
-    virtual ~Cell() { }
+    virtual ~Cell() = default;
 
     bool is_marked() const { return m_mark; }
     void set_marked(bool b) { m_mark = b; }
@@ -55,7 +55,7 @@ public:
     VM& vm() const;
 
 protected:
-    Cell() { }
+    Cell() = default;
 
 private:
     bool m_mark : 1 { false };

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -173,7 +173,7 @@ __attribute__((no_sanitize("address"))) void Heap::gather_conservative_roots(Has
 
 class MarkingVisitor final : public Cell::Visitor {
 public:
-    MarkingVisitor() { }
+    MarkingVisitor() = default;
 
     virtual void visit_impl(Cell& cell) override
     {