overlay: remove navigator input (#288)

## Link to GitHub Issue, if one exists
n/a

## Description of change
No one uses this, remove them to reduce clutter. This is in preparation
to migrate from deprecated ImGui IO model to new one, and this is part
of that.

## Compiling
👍 

## Testing

src/spice2x/games/io.h

@@ -30,12 +30,6 @@ namespace games {
             ScreenResizeScene3,
             ScreenResizeScene4,
             SuperExit,
-            NavigatorActivate,
-            NavigatorCancel,
-            NavigatorUp,
-            NavigatorDown,
-            NavigatorLeft,
-            NavigatorRight,
             HotkeyEnable1,
             HotkeyEnable2,
             HotkeyToggle,