Remove broken badge

Author: errm

README.md

@@ -9,11 +9,8 @@
 
 ## Support
 
-[![Fluentd Slack](http://slack.fluentd.org/badge.svg)](http://slack.fluentd.org/)
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 Join the #plugin-systemd channel on the [Fluentd Slack](http://slack.fluentd.org/)
 
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 ## Requirements
 
 |fluent-plugin-systemd|fluentd|td-agent|ruby|