macos: remove the ability to bind fn/globe

This was recently introduced a few days ago. Unfortunately, this doesn't
work as expected. The "function" modifier is not actually the fn key
but used by macOS to represent a variety of "functional" key presses.
This breaks other bindings such as #2411.

I can't find a source on the internet that reliably tells me how we
can detect fn key presses, but I do find a number of sources that tell
us we can't.

macos/Sources/Ghostty/SurfaceView_AppKit.swift

@@ -743,7 +743,6 @@ extension Ghostty {
             case 0x3B, 0x3E: mod = GHOSTTY_MODS_CTRL.rawValue
             case 0x3A, 0x3D: mod = GHOSTTY_MODS_ALT.rawValue
             case 0x37, 0x36: mod = GHOSTTY_MODS_SUPER.rawValue
-            case 0x3F: mod = GHOSTTY_MODS_FN.rawValue
             default: return
             }