Use libxml2 and libxslt bundled with the gem (with nokogiri 1.11.0+)

With Nokogiri 1.11.0 and later, we can use pre-built library (&5681 (closed)) even on Linux, thus on our CI (#963 (closed)). We can remove libxml2 and libxslt are bundled with the gem.

cf. #846 (closed)