Remove `format: 'cjs'` option from `builder.generateManifest(...)`

[Rich-Harris]

Describe the problem

builder.generateManifest(...) takes a format option which can be either esm or cjs. The cjs option is there because certain adapters needed it at one point. That's no longer the case, at least for the official adapters.

Describe the proposed solution

In the spirit of removing as much technical debt as possible ahead of 1.0, we should remove the cjs option. We should probably do a quick survey of community adapters before ploughing ahead though.

Alternatives considered

Leave it be

Importance

nice to have

Additional Information

No response