[dev-qt/qtcore:5] Don't define QT_ZLIB.

1) It's qtcore, so we can just use whatever the configure generates.
2) The actual macro for the feature is QT(_NO)_COMPRESS. I believe
   QT_ZLIB is defined only when using the bundled zlib.
3) QT(_NO)_ZLIB isn't used anywhere anyway.

Package-Manager: portage-2.2.10