Deprecate and remove gcalclirc mechanism

[dbarnett]

As a follow-up from #416, once the config.toml mechanism is finalized and users have some time to get used to it, we should remove the flag file mechanism and consolidate down to one mechanism for configuring gcalcli.

At that point it will still be possible to pass a flag file like gcalcli @~/.gcalclirc, but it'll be passed explicitly, the files won't be found and used automatically and there won't be any magic names. During the transition it might still be helpful to check if the magic filenames exist and spit out hint messages to users explaining how to pass the same file manually on cli.