function _removeElement(context, element, key) {
    var map = context._entityMaps[key];
    var index = map[element._ID];
    if (index != null) {
        context._indexPools[key].push(index);
        context[key][index] = null;
        map[element._ID] = null;
    }
}
Private helper method to remove an element from a library array.

@method
@private
@param {Object} context Object in possesion of the element arrays.
@param {Object} element The body, force, or constraint to remove.
@param {String} key Where to store the element.
@return {undefined} undefined