function mapCacheDelete(key) {
  var result = getMapData(this, key)['delete'](key);
  this.size -= result ? 1 : 0;
  return result;
}
Removes `key` and its value from the map.

@private
@name delete
@memberOf MapCache
@param {string} key The key of the value to remove.
@returns {boolean} Returns `true` if the entry was removed, else `false`.