ci: Drop buildpod configuration limitng on label and namespace

Also drop idleMinutes as suggested by build team.
This change should fix a problem keeping internal build pods from being
spawned for release builds

.ci/releasePipeline.groovy

@@ -1,12 +1,9 @@
 pipeline {
     agent {
         kubernetes {
-            label 'ca-jenkins-agent'
             cloud 'linux-amd64'
-            namespace 'keptn-jenkins-slaves-ni'
             nodeSelector 'kubernetes.io/arch=amd64,kubernetes.io/os=linux'
             instanceCap '2'
-            idleMinutes '2'
             yamlFile '.ci/jenkins_agents/ca-jenkins-agent.yaml'
         }
     }