Smart_open: Drop support for host/port in S3 URLs

[mpenkov]

What about dropping support for host/port in S3 URLs (endpoint_url may be still provided in transport_params)? It seems this is rarely used feature and these URLs also break RFC rules (like two @ characters in url). Parsing will be much easier without that (parse.urlsplit will be enough).

_Originally posted by @mrk-its in https://github.com/RaRe-Technologies/smart_open/issues/378#issuecomment-551143613_

@piskvorky what do you think?

[piskvorky]

I never used that functionality, so no preference either way. Clean and consistent is my general preference – or we'll drown in corner cases.