Revert "codegen: Add go:generate to command line in output"

[camh-]

Remove the //go:generate from the template as it has a number of issues:

• It breaks wbnf itself because the template itself triggers go generate.
• It breaks wbnf itself because it uses a different command to generate its own codegen.
• It is not always correct - it only works if the output file is in the same directory as where wbnf is run.

Turns out this was a bad idea, so revert it.

This reverts commit 2ac1f0c4c25f741407c6f1bd7dde166e1a7a9a56.

Fixes: #61