WebWorker: Reuse main thread VM for DedicatedWorker realms

While creating a new VM feels warm and fuzzy from an isolation
perspective, having multiple JS heaps in the same process is a footgun
waiting to happen. Additionally, there are still many places in LibWeb
that reach for the main thread VM to check for the current realm to do
things, such as Web::HTML::incumbent_settings_object().

Userland/Services/WebWorker/DedicatedWorkerHost.h

@@ -8,7 +8,6 @@
 
 #include <AK/RefCounted.h>
 #include <AK/URL.h>
-#include <LibJS/Runtime/VM.h>
 #include <LibWeb/Bindings/MainThreadVM.h>
 #include <LibWeb/Forward.h>
 
@@ -22,7 +21,6 @@ public:
     void run();
 
 private:
-    NonnullRefPtr<JS::VM> m_worker_vm;
     RefPtr<Web::HTML::WorkerDebugConsoleClient> m_console;
     Web::Page& m_page;