Remove OS X unsupported warning.

No other platform has such a warning and it really is not important.

Change-Id: I70185dff6b0fad11c7a767f787679dc53d04ecc7
(backported from qtbase/cd17b500a85b34c16ff8f4212cf2ff35715040ba)
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>