Drop unencrypted http2 (h2c) support.

[jcoyne]

Web browsers don't support this. It's less complex if we don't need to do this either.

[DiegoPino]

@jcoyne makes sense. I double checked and if you run nginx you can not proxy via http2 so also proxied use case seems to be solved. Even if the http2 specs do allow unencrypted communications. Should we ask other type of front end users? But I am totally fine with dropping it! Should we document that if running Cantaloupe proxied, we will only support 1.1?