`decode_body`: Drop automatic decoding of CSV

[wojtekmach]

This is too confusing that depending on the presence of :nimble_csv we have or not have automatic CSV decoding.

This is different from optionally depending on ezstd/brotli because if we don't have these we don't send them in accept-encoding and so the server shouldn't use them.

This is a breaking change scheduled for Req v1.0.