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/mda-lmtp/mda-lmtp.go

@@ -1,9 +1,6 @@
 // mda-lmtp is a very basic MDA that uses LMTP to do the delivery.
 //
 // See the usage below for details.
-//
-//go:build !coverage
-
 package main
 
 import (