Remove support to MIX_GIT_FORCE_HTTPS environment option

This used to be useful in case the git port was blocked.
Merge pull request #2104 from rcillo/rd-remove-git-force-https-option

Remove support to MIX_GIT_FORCE_HTTPS environment option
Deprecate GenFSM

We plan to provide better abstractions around GenServer and GenEvent
in upcoming releases but we won't do so for GenFSM. There are other
interesting approaches out there, like `plain_fsm`, which are easier
to understand and use than OTP's FSM.