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/Runtime/BigInt.h

@@ -14,7 +14,7 @@ namespace JS {
 class BigInt final : public Cell {
 public:
     explicit BigInt(Crypto::SignedBigInteger);
-    virtual ~BigInt();
+    virtual ~BigInt() override = default;
 
     const Crypto::SignedBigInteger& big_integer() const { return m_big_integer; }
     const String to_string() const { return String::formatted("{}n", m_big_integer.to_base(10)); }