Cleanup obsolete hand-coded XContent duplicate checks

[danielmitterdorfer]

With #22073 and #22225 we enabled strict duplicate checks for XContent. Although our benchmarks only showed a negligible performance drop of 1 - 2% (see #19614), Elasticsearch 6.0 still allows to disable the strict duplicate checks with an undocumented flag. Hence, we also needed to keep the existing hand-coded duplicate checks around in case Elasticsearch is started with disabled XContent parser duplicate checks.

In Elasticsearch 7.0, we should remove the undocumented fallback to disable strict duplicate checks and also remove our hand-coded duplicate checks.