refactor: do not return error as it is always nil

[alexandear]

The functions brotliBody, gzipBody, deflateBody, and zstdBody always return a nil error. Therefore, this PR removes error from the return argument of these functions.

[erikdubbelboer]

I wonder if it's better to remove the error return or to actually capture the errors in the NewStreamReader callback that are currently being ignored. Although I'm not sure how to capture those without causing garbage to be generate.

[erikdubbelboer]

Lets merge this for now. Thanks!