Remove manifest section

[myrzakereyms]

The manifest section was removed in (wpack-wg/bundled-responses#6) from the spec, however we still have it implemented both in Go tools and chromium. Should this be removed completely?

[hayatoito]

Regarding Go tools, I think it is okay to remove the support of manifest section in b2.

Re chromium, it would be better to file an issue on crbug.

[hayatoito]

[Issue Triage]
It seems Go tools still support a manifest section. We might want to remove it.