Apply more mechanical simplifications using recent Go features

This is a follow-up on the `go fix` run in commit
0210c5fd92.

That commit recorded the automated updates.

This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.

It also removes some now-obsolete comments and statements.

test/util/conngen/conngen.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // SMTP connection generator, for testing purposes.
 package main
 
@@ -8,7 +6,6 @@ import (
 	"net"
 	"net/http"
 	"net/smtp"
-	"time"
 
 	_ "net/http/pprof"
 
@@ -66,9 +63,7 @@ func main() {
 	log.Infof("done, created %d simultaneous connections", *count)
 
 	if *wait {
-		for {
-			time.Sleep(24 * time.Hour)
-		}
+		select {}
 	}
 
 	for _, c := range conns {