Remove PPnet, which is no longer maintained since 3/2018.

README.md

@@ -437,7 +437,6 @@ _[IPBX](https://en.wikipedia.org/wiki/IP_PBX) telephony software_
 - [Oxwall](http://www.oxwall.org/) - Oxwall is used for a wide range of projects starting from family sites and custom social networks to collaboration tools and enterprise community solutions. ([Source Code](https://bitbucket.org/oxwall/public)) `CPAL-1.0` `PHP`
 - [Patchwork](https://github.com/ssbc/patchwork) - Decentralized messaging and sharing app built on top of Secure Scuttlebutt (SSB). `AGPL-3.0-only` `JavaScript`
 - [phpBB](https://www.phpbb.com/) - Flat-forum bulletin board software solution that can be used to stay in touch with a group of people or can power your entire website. ([Source Code](https://github.com/phpbb/phpbb)) `GPL-2.0` `PHP`
-- [PPnet](https://github.com/pixelpark/ppnet) - Create and host your own social network. `MIT` `Javascript`
 - [Pump.io](http://pump.io/) - Stream server that does most of what people really want from a social network. ([Source Code](https://github.com/e14n/pump.io)) `Apache-2.0` `Nodejs`
 - [Socialhome](https://socialhome.network) - Federated and decentralized profile builder and social network engine. ([Demo](https://socialhome.network/public/), [Source Code](https://github.com/jaywink/socialhome)) `AGPL-3.0` `Python`
 - [Symphony](https://hacpai.com/) - Modern community (forum/SNS/blog) platform written in Java. ([Source Code](https://github.com/b3log/symphony)) `GPL-3.0` `Java`