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.idl

@@ -1,5 +1,11 @@
+#import <DOM/Event.idl>
+
 interface SubmitEvent : Event {
+    constructor(DOMString type, optional SubmitEventInit eventInitDict = {});
 
     readonly attribute HTMLElement? submitter;
-
+};
+
+dictionary SubmitEventInit : EventInit {
+   HTMLElement? submitter = null;
 };