qfilesystemengine_mac.cpp is empty => purge

All mac related qfilesystemengine operations are done in
qfilesystemengine_unix.
Purging it like commit bd7ca33889139782f3f0063f93ca9c1f39501a17 in
qtbase

Change-Id: I16ba494b699d731c3cd688cbd34b81cc67851b47
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>