function removeBillboards(billboardCollection) {
  if (billboardCollection._billboardsRemoved) {
    billboardCollection._billboardsRemoved = false;

    const newBillboards = [];
    const billboards = billboardCollection._billboards;
    const length = billboards.length;
    for (let i = 0, j = 0; i < length; ++i) {
      const billboard = billboards[i];
      if (defined(billboard)) {
        billboard._index = j++;
        newBillboards.push(billboard);
      }
    }

    billboardCollection._billboards = newBillboards;
  }
}
Removes all billboards from the collection.

@performance <code>O(n)</code>.  It is more efficient to remove all the billboards
from a collection and then add new ones than to create a new collection entirely.

@exception {DeveloperError} This object was destroyed, i.e., destroy() was called.


@example
billboards.add(...);
billboards.add(...);
billboards.removeAll();

@see BillboardCollection#add
@see BillboardCollection#remove