refactor(gui): drop setFullscreen now that chrome is a Lua node

Fullscreen was the firmware surrendering a strip it clipped apps out of.
The strip is a sibling node now, so an app that wants the panel is chrome
choosing not to build itself.

lua/api/core/gui.lua

@@ -107,10 +107,6 @@ function gui.fillCircle(x, y, radius, color, background) end
 ---@param border? GuiColor Omitted for no border.
 function gui.roundRect(x, y, w, h, radius, background, top, bottom, border) end
 
----Temporarily gives the app the full panel, including firmware chrome.
----@param on boolean
-function gui.setFullscreen(on) end
-
 ---Fills a polygon.
 ---@param xs integer[]
 ---@param ys integer[]