irule remove "www."

From an SEO perspective, a 301 redirect would be best;-

when HTTP_REQUEST {
   if {[string tolower [HTTP::host]] starts_with "www." } {
      HTTP::respond 301 Location "https://[substr [HTTP::host] 4][HTTP::uri]" content "" noserver
      return
   }
}

Alternatively, if you want the browser to continue to display the www. Host variant, you may want to proxy the request with a modified Host header rather than redirecting;

when HTTP_REQUEST {
   if {[string tolower [HTTP::host]] starts_with "www." } {
      HTTP::host [substr [HTTP::host] 4]
   }
}