_deleteCoverages(imageryLayer, coverages) {
    if (imageryLayer.isDestroyed()) {
      return;
    }
    const length = coverages.length;
    for (let i = 0; i < length; i++) {
      const coverage = coverages[i];
      const imagery = coverage.imagery;
      imagery.releaseReference();
    }
  }
Delete the given imagery coverage objects for the given imagery
layer, meaning that it will cause the reference counters of the
imageries to be decreased.

If the imagery layer already has been destroyed, then nothing
will be done.

@param {ImageryLayer} imageryLayer The imagery layer
@param {ImageryCoverage[]} coverages The coverages