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.h

@@ -25,8 +25,6 @@
 
 namespace JS {
 
-Realm& realm_from_global_object(GlobalObject&);
-
 class Heap {
     AK_MAKE_NONCOPYABLE(Heap);
     AK_MAKE_NONMOVABLE(Heap);