www/serendipity: Update from 2.0.5 to 2.1.1

Changelog: https://blog.s9y.org/archives/273-Serendipity-2.1.0-released.html
Changelog: https://blog.s9y.org/archives/274-Serendipity-2.1.1-released.html

Also remove no longer needed dependency.

PR: 218526
Submitted by: Jochen Neumeister <joneum@bsdproject.de>