Removing support for Carthage dependency manager

[icanzilb]

Few people have tried to help adding Carthage support but the issue is more complex than anyone so far has been able to address.

RxTimelane is a multi-platform package that depends on another multiplatform package RxSwift - thus the shared scheme that adds the Carthage support needs to be able to build frameworks with proper multi-platform dependencies.

I've looked into prior art and it seems the setup here is one that is working for the above described situation: https://github.com/RxSwiftCommunity/RxAlamofire

I would rather remove the still broken Carthage support before anyone adds a working PR.