Removed Matrix room references

It was seldom used, point to GitHub discussions instead.

CONTRIBUTING.md

@@ -29,8 +29,8 @@ with the e-mail or use a chat. If you suspect that the issue may not be always
 replicable, you can use a script `ultragrid-bugreport-collect.sh` to collect
 data about a computer and attach its result. See also [here](doc/REPORTING-BUGS.md).
 
-Especially for various questions about UltraGrid usage you may also use our Matrix chat
-[![@ultragrid:matrix.org](https://img.shields.io/badge/Matrix-chat-black)](https://matrix.to/#/!IrTYOLJOmZIoTBiITI:matrix.org?via=matrix.org).
+For general questions, ideas or remarks you are encountered to use the project's
+[GitHub Discussions](https://github.com/CESNET/UltraGrid/discussions).
 
 ## Feature requests
 Feature requests can be submitted with the same channels as bugs.

README.md

@@ -3,7 +3,6 @@ UltraGrid - A High Definition Collaboratory
 [![Facebook Follow](https://img.shields.io/badge/Facebook-follow-blue)](https://www.facebook.com/UltraGrid/)
 [![Mastodon Follow](https://img.shields.io/badge/Mastodon-follow-blue)](https://mastodon.technology/@UltraGrid)
 [![Twitter Follow](https://img.shields.io/badge/Twitter-follow-blue)](https://twitter.com/UltraGrid_CZ)
-[![@ultragrid:matrix.org](https://img.shields.io/badge/Matrix-chat-black)](https://matrix.to/#/!IrTYOLJOmZIoTBiITI:matrix.org?via=matrix.org)
 [![Web Visit](https://img.shields.io/badge/web-visit-orange)](http://www.ultragrid.cz)
 [![Coverity Scan Build Status](https://scan.coverity.com/projects/2851/badge.svg)](https://scan.coverity.com/projects/2851)
 [![C/C++ CI](../../workflows/C%2FC%2B%2B%20CI/badge.svg)](../../actions)