ci: use only built-in pipeline steps

The Jenkins the job runs on has no AnsiColor plugin, so the pipeline
failed to compile. Drop ansiColor and use the built-in deleteDir()
instead of the Workspace Cleanup plugin's cleanWs().

ci/jenkins/README.md

@@ -91,8 +91,6 @@ The job needs these plugins:
 - GitHub (for the push trigger)
 - Credentials Binding
 - Timestamper
-- AnsiColor
-- Workspace Cleanup
 
 Most are part of the suggested set.