Bug 1395146 - part1: Get rid of nsITextControlElement::IsPlainTextControl() and nsTextEditorState::IsPlainTextEditor() r=smaug

nsTextEditorState::GetValue() refers nsITextControlElement::IsPlainTextControl()
via nsTextEditorState::IsPlainTextEditor().  However, it always returns true and
virtual call with QI.  So, we should get rid of these unnecessary methods.

MozReview-Commit-ID: 3gHdGrzlys4

dom/html/HTMLInputElement.cpp

@@ -7402,13 +7402,6 @@ HTMLInputElement::IsTextArea() const
   return false;
 }
 
-NS_IMETHODIMP_(bool)
-HTMLInputElement::IsPlainTextControl() const
-{
-  // need to check our HTML attribute and/or CSS.
-  return true;
-}
-
 NS_IMETHODIMP_(bool)
 HTMLInputElement::IsPasswordTextControl() const
 {