_deleteCoveragesPerLayer() {
    const coveragesPerLayer = this._coveragesPerLayer;
    if (!defined(coveragesPerLayer)) {
      return;
    }
    const imageryLayers = this._currentImageryLayers;
    const length = coveragesPerLayer.length;
    for (let i = 0; i < length; i++) {
      const imageryLayer = imageryLayers[i];
      const coverages = coveragesPerLayer[i];
      this._deleteCoverages(imageryLayer, coverages);
    }
    delete this._currentImageryLayers;
    delete this._coveragesPerLayer;
  }
Delete the <code>_coveragesPerLayer</code> if they are defined.

This will call <code>deleteCoverages</code> for each set of coverages,
and eventually delete the <code>_coveragesPerLayer</code>.

This will cause the reference counters of the imageries to be
decreased.