ci: Remove Clean parameter

This parameter no longer makes sense as the workspace is not preserved
between runs anyhow.

ci/Jenkinsfile

@@ -11,20 +11,9 @@ pipeline {
         disableConcurrentBuilds()
     }
 
-    parameters {
-        booleanParam \
-            name: 'Clean',
-            description: 'Clean the workspace and perform a full rebuild'
-    }
-
     stages {
         stage('Build') {
             steps {
-                script {
-                    if (params.Clean) {
-                        sh 'rm -rf _build'
-                    }
-                }
                 copyArtifacts \
                     filter: 'aarch64/mqttdpms',
                     projectName: '../mqttdpms/master',