LibWeb: Add the missing SubmitEvent IDL constructor

This commit also removes the SubmitEvent.cpp file, as all of the method
implementations were trivial and could be inlined into the header file.

Userland/Libraries/LibWeb/HTML/SubmitEvent.cpp

@@ -1,32 +0,0 @@
-/*
- * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
- *
- * SPDX-License-Identifier: BSD-2-Clause
- */
-
-#include <LibWeb/HTML/HTMLElement.h>
-#include <LibWeb/HTML/SubmitEvent.h>
-
-namespace Web::HTML {
-
-NonnullRefPtr<SubmitEvent> SubmitEvent::create(const FlyString& event_name, RefPtr<HTMLElement> submitter)
-{
-    return adopt_ref(*new SubmitEvent(event_name, move(submitter)));
-}
-
-SubmitEvent::SubmitEvent(const FlyString& event_name, RefPtr<HTMLElement> submitter)
-    : DOM::Event(event_name)
-    , m_submitter(submitter)
-{
-}
-
-SubmitEvent::~SubmitEvent()
-{
-}
-
-RefPtr<HTMLElement> SubmitEvent::submitter() const
-{
-    return m_submitter;
-}
-
-}