Remove references to Terminal

!382 (merged) replaced all the references to Terminal with Channel.

Nevertheless, to keep backward compatibility, we needed to keep the TerminalSettings object and add a translation from TerminalSettings to ChannelSettings.

Specifically, the following actions have to be taken:

• Remove internal/api/terminal_settings.go file
• In the object api.Reponse remove the field Terminal
• In internal/channel/channel.go remove the following block inside the PreAuthorizationHandler:

// Used during the transition from Terminal to Channel
// Once we remove the TerminalSettings object we can remove
// this condition
if a.Terminal != nil {
  a.Channel = a.Terminal.Channel()
}

• In channel_test.go remove the methods wireupTerminal, terminalOkBody, and TestChannelHappyPathWithTerminalResponse.