[escher] Clean Field and Input class hierarchy:

Class hierarchy:
- InputEventHandler
- TextInput
- EditableField derives from InputEventHandler
- LayoutField, TextField and ExpressioField derives from EditableField
- TextArea and TextField derives from TextInput

escher/src/text_input.cpp

@@ -59,13 +59,6 @@ TextInput::TextInput(Responder * parentResponder, View * contentView) :
 {
 }
 
-Toolbox * TextInput::toolbox() {
-  if (delegate()) {
-    return delegate()->toolboxForTextInput(this);
-  }
-  return nullptr;
-}
-
 bool TextInput::removeChar() {
   contentView()->removeChar();
   scrollToCursor();