Bug 1683226 - part 2: Make HTMLEmbedElement and HTMLObjectElement stop overriding nsIContent::GetDesiredIMEState() r=smaug

Currently, they are never focusable when its type is "plugin".
So, making stop them returning `IMEEnabled::Plugin` won't change
anything, but it guarantees that nobody will see `IMEEnabled::Plugin`
at runtime.  This is a preparation for the following patches.

Differential Revision: https://phabricator.services.mozilla.com/D100101

dom/html/HTMLObjectElement.cpp

@@ -337,14 +337,6 @@ bool HTMLObjectElement::IsHTMLFocusable(bool aWithMouse, bool* aIsFocusable,
   return false;
 }
 
-nsIContent::IMEState HTMLObjectElement::GetDesiredIMEState() {
-  if (Type() == eType_Plugin) {
-    return IMEState(IMEEnabled::Plugin);
-  }
-
-  return nsGenericHTMLFormElement::GetDesiredIMEState();
-}
-
 NS_IMETHODIMP
 HTMLObjectElement::Reset() { return NS_OK; }