Revert "what if we leave ongui only in the base class"

This reverts commit 718c23b.

Author: K-Tone

Packages/com.unity.inputsystem/InputSystem/Editor/InputParameterEditor.cs

@@ -27,13 +27,6 @@ public abstract class InputParameterEditor
         /// </summary>
         public object target { get; internal set; }
 
-        /// <summary>
-        /// Callback for implementing a custom UI.
-        /// It was mostly needed for the imgui-based editors and should be avoided now.
-        /// Should be removed once we can release the next major version.
-        /// </summary>
-        public void OnGUI() {}
-
         /// <summary>
         /// Add visual elements for this parameter editor to a root VisualElement.
         /// </summary>