Make HarfBuzz-NG the default shaper on all platforms

[ChangeLog][Important Behavior Changes] HarfBuzz-NG is now the default
shaper on all platforms. This results in a better shaping results
for various languages, better performance, and lower memory consumption.

Task-number: QTBUG-18980
Change-Id: I4d9454fc37e9050873df3857e52369dfc7f191b2
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>