remove link to spectrum (#183)

Signed-off-by: Nicolas Rol <[email protected]>

Author: rolnico

Diff for: README.md

@@ -4,7 +4,6 @@
 [![Coverage Status](https://sonarcloud.io/api/project_badges/measure?project=com.powsybl%3Apowsybl-metrix&metric=coverage)](https://sonarcloud.io/component_measures?id=com.powsybl%3Apowsybl-metrix&metric=coverage)
 [![Quality Gate](https://sonarcloud.io/api/project_badges/measure?project=com.powsybl%3Apowsybl-metrix&metric=alert_status)](https://sonarcloud.io/dashboard?id=com.powsybl%3Apowsybl-metrix)
 [![MPL-2.0 License](https://img.shields.io/badge/license-MPL_2.0-blue.svg)](https://www.mozilla.org/en-US/MPL/2.0/)
-[![Join the community on Spectrum](https://withspectrum.github.io/badge/badge.svg)](https://spectrum.chat/powsybl)
 [![Slack](https://img.shields.io/badge/slack-powsybl-blueviolet.svg?logo=slack)](https://join.slack.com/t/powsybl/shared_invite/zt-rzvbuzjk-nxi0boim1RKPS5PjieI0rA)