Deletes an entry in the `SjlMap`.
@method sjl.stdlib.SjlMap#delete
@param key {*} - Key of key-value pair to remove.
@returns {SjlMap}
function (key) {
                if (this.has(key)) {
                    var _index = this._keys.indexOf(key);
                    this._values.splice(_index, 1);
                    this._keys.splice(_index, 1);
                }
                return this;
            }