Revert "Add rebar3-hex plugin"

[kostis]

Reverts #213

[kostis]

Continuing the discussion of #213, I will revert this.

@ferd Should I also add a {project_plugins, [rebar3_hex]} to rebar.config or not? (This part is not so clear to me.)

Kostis, who is (slowly) entering the brave rebar3 world...

[ferd]

No need to, that plugin is something I tend to maintain globally.

Project plugins are more useful for example when people install the rebar3 plugin for proper I have written (https://github.com/ferd/rebar3_proper) which lets you expose the rebar3 proper command when using your app at the root, but won't include it when the app is included as a dep (and therefore won't make rebar3 proper available to the app that includes the dep that uses PropEr)