Remove useless DARWIN_NO_CARBON define from uikit mkspecs

This was used solely for building freetype. It was made obsolete as of
2eaf0cf8fd which upgraded freetype to a
newer version which does not rely on that define, first included in 5.5.

Change-Id: Iaaea8d6783032d784f0a370f8404972967fa7a06
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

mkspecs/common/uikit.conf

@@ -6,6 +6,5 @@ QMAKE_PLATFORM         += uikit
 CONFIG                 += bitcode reduce_exports shallow_bundle no_qt_rpath
 
 INCLUDEPATH            += $$PWD/uikit
-DEFINES                += DARWIN_NO_CARBON
 
 include(mac.conf)