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/generate_cert/generate_cert.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // Utility to generate self-signed certificates.
 // It generates a self-signed x509 certificate and key pair, and writes them
 // to "fullchain.pem" and "privkey.pem".