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.cpp

@@ -17,10 +17,6 @@ BigInt::BigInt(Crypto::SignedBigInteger big_integer)
     VERIFY(!m_big_integer.is_invalid());
 }
 
-BigInt::~BigInt()
-{
-}
-
 BigInt* js_bigint(Heap& heap, Crypto::SignedBigInteger big_integer)
 {
     return heap.allocate_without_global_object<BigInt>(move(big_integer));