Remove QT_HARFBUZZ=old checks

The "old" Harfbuzz engine has been removed from Qt as the new one is superior and the failsafe is no longer needed. But the QT_HARFBUZZ=old environment variable is still read. Setting this now leads to text missing entirely. This can easily cause confusion when porting old systems where this was set as a work-around for missing features in the new Harfbuzz, so we should remove all mentions of it and just ignore it.