Travis CI: remove HarfBuzz and re-remove Fontconfig from OS X builds

Travis CI: remove HarfBuzz and re-remove Fontconfig from OS X builds

Installing HarfBuzz through Homebrew seems to be consistently slow
whether we use the bottles and disable the Fontconfig cache or build
it from source and drop Fontconfig and other dependencies entirely.

To speed up OS X builds, disable both HarfBuzz and Fontconfig.
We build with HarfBuzz and Fontconfig on Linux, and we should
not have any platform-dependent code that depends on them,
so this should not reduce our code coverage.