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/t-03-queue_persistency/addtoqueue.go

@@ -5,7 +5,6 @@
 // daemon for testing purposes only.
 //
 //go:build ignore
-// +build ignore
 
 package main