macOS: Remove reference to building for Carbon

Qt hasn't had an option to build for Carbon for a long time.

Change-Id: I7d1fe4e1fcdbf00455f36aa097dd5df7d4e1d567
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>