Revert "plugins: add matrix-communications"

reverts commit 611be55.
We are instead using the webhook for maubot.

See coreos/coreos-ci#55 (comment)

Author: jbtrystram

jenkins/controller/plugins.txt

@@ -32,7 +32,6 @@ splunk-devops-extend:1.10.1
 splunk-devops:1.10.1
 antisamy-markup-formatter:162.v0e6ec0fcfcf6
 jms-messaging:1.1.27
-matrix-communication:13.vdcfb_b_44ce852
 
 # The below list are plugins that are also in base-plugins.txt but for
 # some reason or other we need to temporarily freeze or fast-track.

utils.groovy

@@ -710,12 +710,13 @@ def get_supported_additional_arches() {
 def matrixSend(message) {
 
     withCredentials([usernamePassword(credentialsId: 'matrix-bot-webhook-token',
-                                      usernameVariable: 'URL',
+                                      usernameVariable: 'MATRIX_WEBHOOK_URL',
                                       passwordVariable: 'TOKEN')]) {
 
     shwrap("""
            curl -X POST -H "Content-Type: application/json" \
-           -u $TOKEN $URL \
+           -u $TOKEN $MATRIX_WEBHOOK_URL \
+           --silent \
            -d '
            {
              "body": "$message"