Remove OSX 10.6 builds from Qt5 CI

Change-Id: I839e6eb92a59ed478eb49a2b2299f57fb174c1f8
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Jani Heikkinen <jani.heikkinen@digia.com>
Reviewed-by: Tony Sarajärvi <tony.sarajarvi@digia.com>