Bug 1717156 - part 2: Get rid of nsIEditor::eEditorWidgetMask and its accessor r=m_kato

With the previous patch, we know `nsIEditor::eEditorWidgetMask` always
matches with `EditorBase::IsTextEditor()`.  And it's not referred from JS
(including comm-central and BlueGriffon).  So, we can get rid of it.

Differential Revision: https://phabricator.services.mozilla.com/D118262

dom/html/TextControlState.cpp

@@ -1707,9 +1707,6 @@ nsresult TextControlState::PrepareEditor(const nsAString* aValue) {
     editorFlags |= nsIEditor::eEditorPasswordMask;
   }
 
-  // All nsTextControlFrames are widgets
-  editorFlags |= nsIEditor::eEditorWidgetMask;
-
   // Spell check is diabled at creation time. It is enabled once
   // the editor comes into focus.
   editorFlags |= nsIEditor::eEditorSkipSpellCheck;