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.

cmd/smtp-check/smtp-check.go

@@ -1,7 +1,4 @@
 // smtp-check is a command-line too for checking SMTP setups.
-//
-//go:build !coverage
-
 package main
 
 import (