Remove matrix reference

The main communications platform is still Libera.chat, matrix was only
provided by the hosted IRC-Matrix bridge. The bridge was turned off
already and won't come back.

README.md

@@ -11,7 +11,6 @@ Officially hosted instance: https://rss-bridge.org/bridge01/
 [![LICENSE](https://img.shields.io/badge/license-UNLICENSE-blue.svg)](UNLICENSE)
 [![GitHub release](https://img.shields.io/github/release/rss-bridge/rss-bridge.svg?logo=github)](https://github.com/rss-bridge/rss-bridge/releases/latest)
 [![irc.libera.chat](https://img.shields.io/badge/irc.libera.chat-%23rssbridge-blue.svg)](https://web.libera.chat/#rssbridge)
-[![Chat on Matrix](https://matrix.to/img/matrix-badge.svg)](https://matrix.to/#/#rssbridge:libera.chat)
 [![Actions Status](https://img.shields.io/github/actions/workflow/status/RSS-Bridge/rss-bridge/tests.yml?branch=master&label=GitHub%20Actions&logo=github)](https://github.com/RSS-Bridge/rss-bridge/actions)
 
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