hcl2_upgrade: fix liniting issues with fmt.Sprintf
hcl2_upgrade: fix liniting issues with fmt.Sprintf

Some literal strings without format-string directives nor arguments were
littered around the hcl2_upgrade command's code, which is pointed out by
the linters.

Since they serve no purpose, we remove them.

hashicorp/packer,hashicorp/packer,hashicorp/packer,hashicorp/packer