function removeMarkedExtensions() {
        return Async.doInParallel_aggregateErrors(
            Object.keys(_idsToRemove),
            function (id) {
                return remove(id);
            }
        );
    }
Removes extensions previously marked for removal.
@return {$.Promise} A promise that's resolved when all extensions are removed, or rejected
    if one or more extensions can't be removed. When rejected, the argument will be an
    array of error objects, each of which contains an "item" property with the id of the
    failed extension and an "error" property with the actual error.