LibJS: Remove GlobalObject from VM::throw_completion()

This is a continuation of the previous five commits.

A first big step into the direction of no longer having to pass a realm
(or currently, a global object) trough layers upon layers of AOs!
Unlike the create() APIs we can safely assume that this is only ever
called when a running execution context and therefore current realm
exists. If not, you can always manually allocate the Error and put it in
a Completion :^)

In the spec, throw exceptions implicitly use the current realm's
intrinsics as well: https://tc39.es/ecma262/#sec-throw-an-exception

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -349,11 +349,4 @@ void Heap::uproot_cell(Cell* cell)
     m_uprooted_cells.append(cell);
 }
 
-// Temporary helper function as we can't pass a realm directly until Heap::allocate<T>() and VM::throw_completion<T>() receive one.
-// Heap.h and VM.h only have a forward declaration of the GlobalObject, so no inlined member access possible.
-Realm& realm_from_global_object(GlobalObject& global_object)
-{
-    return *global_object.associated_realm();
-}
-
 }