channel/user/remove - Removes an added user from a channel.

Method parameters:
id - The ID of the channel remove the user from.
username - The Zvini username of the user to remove.

Returns a JSON document of the following format:
<boolean> - Always true.

Expected errors:
CHANNEL_NOT_FOUND - A channel with the ID doesn't exist.
ENTER_USERNAME - The username is empty.
USER_NOT_FOUND - No such user with the username.
USER_NOT_ADDED - The user is not added to the channel.