Bug 1729342 - Use delegatesFocus for <input type=date/time/datetime-local> widget r=emilio

Currently datetimebox uses some hardcoded logic to find the edit
field inside the shadow tree and focus it.

Since we have delegatesFocus implemented, we could just use it without
these logic.

Differential Revision: https://phabricator.services.mozilla.com/D125440

dom/html/HTMLInputElement.h

@@ -134,9 +134,6 @@ class HTMLInputElement final : public TextControlElement,
 
   int32_t TabIndexDefault() override;
   using nsGenericHTMLElement::Focus;
-  void Blur(ErrorResult& aError) override;
-  void Focus(const FocusOptions& aOptions, CallerType aCallerType,
-             ErrorResult& aError) override;
 
   // nsINode
 #if !defined(ANDROID) && !defined(XP_MACOSX)