fix subscribe links, remove disabled icons (apache#12474)

* fix subscribe links, remove disabled icons

* update slack channel

docs/community/mxnet_channels.md

@@ -2,9 +2,9 @@
 
 Converse with the MXNet community via the following channels:
 
-- [Forum](https://discuss.mxnet.io/): [discuss.mxnet.io](https://discuss.mxnet.io/) <i class="fas fa-external-link-alt"></i>
-- [MXNet Apache developer mailing list](https://lists.apache.org/[email protected]) ([email protected]): To subscribe, send an email to <a href="mailto:[email protected]">dev[email protected]</a> <i class="far fa-envelope"></i>
-- [MXNet Apache user mailing list](https://lists.apache.org/[email protected]) ([email protected]): To subscribe, send an email to <a href="mailto:[email protected]">user[email protected]</a> <i class="far fa-envelope"></i>
-- [MXNet Slack channel](https://apache-mxnet.slack.com): To request an invitation to the channel please subscribe to the mailing list above and then email: <a href="mailto:[email protected]">[email protected]</a> <i class="far fa-envelope"></i>
+- [Forum](https://discuss.mxnet.io/): [discuss.mxnet.io](https://discuss.mxnet.io/)
+- [MXNet Apache developer mailing list](https://lists.apache.org/[email protected]) ([email protected]): To subscribe, send an email to <a href="mailto:[email protected]">user[email protected]</a>
+- [MXNet Apache user mailing list](https://lists.apache.org/[email protected]) ([email protected]): To subscribe, send an email to <a href="mailto:[email protected]">dev[email protected]</a>
+- [MXNet Slack channel](https://the-asf.slack.com/) (Channel: #mxnet): To request an invitation to the channel please subscribe to the mailing list above and then email: <a href="mailto:[email protected]">[email protected]</a>
 
 Note: if you have an email address with apache.org, you do not need an approval to join the MXNet Slack channel.