[API] Remove unused: optional_inputs & outputs (#597)

Code search shows zero usage. If kept, typing properly would require
effort.

Removed:
- `LGraphNode.optional_inputs`
- `LGraphNode.optional_outputs`

src/LGraphNode.ts

@@ -279,10 +279,6 @@ export class LGraphNode implements Positionable, IPinnable, IColorable {
   _shape?: RenderShape
   mouseOver?: IMouseOverData
   redraw_on_mouse?: boolean
-  // Appears unused
-  optional_inputs?
-  // Appears unused
-  optional_outputs?
   resizable?: boolean
   clonable?: boolean
   _relative_id?: number