lastModifiedDate should be removed

[jimmywarting]

https://github.com/apache/cordova-plugin-file/blob/5274cab39e3f96a8682a74ba28663f32611501c8/www/File.js#L37

File.lastModifiedDate is deprecated and also removed in FF & Safari

You should stop supporting it also and remove it (or at least throw a error message saying it's deprecated in a getter method)