Cleanup exported types in connection

[adizere]

The exported module brings up a related problem I believe. Parts of this module (and its intended functionality) may be unnecessary and we can remove it..

yes, we should remove the traits from the exported.rs files (in both connection and channel folders).

Originally posted by @ancazamfir in informalsystems/hermes#186 (comment)

More specifically, we can remove the traits Connection and ConnectionCounterparty:

https://github.com/informalsystems/ibc-rs/blob/574371a280a282bdfaa1045ca8a6e628d612b79d/modules/src/ics03_connection/exported.rs#L6-L25