Remove old build tag comments

Go is transitioning from the old '// +build' form to '//go:build', this
removes all uses of the old form, adding the new one where needed.

See https://go.googlesource.com/proposal/+/master/design/draft-gobuild.md

internal/client/integration_test.go

@@ -2,7 +2,6 @@
 // Copyright (C) 2020 by Red Hat, Inc.
 
 //go:build integration
-// +build integration
 
 package client
 

internal/client/unit_test.go

@@ -2,7 +2,6 @@
 // Copyright (C) 2020 by Red Hat, Inc.
 
 //go:build !integration
-// +build !integration
 
 package client