Remove mentions to unless/2 in the case/cond/if guide

[sabiwara]

I think it is actually a nice simplification since it's one less thing in the getting started.

I'm also trying to rewrite the macro guide which is using unless as an example, I'm not sure what's the best candidate:

• if: less trivial and a tad verbose because will need to write it in terms of case
• &&: should be straightforward to change the existing guide, and can easily be implemented in terms of if (but maybe less "interesting/impressive" than implementing if?)
• keep unless?
• something else?

[josevalim]

I think the guide can still use unless and we mention it is an exercise? Although implementing the operator && would also work.

[sabiwara]

I think the guide can still use unless and we mention it is an exercise?

Yeah that would work, the only part we'd lose is

In fact, unless/2 in Elixir is implemented as a macro:

or perhaps we can say "used to have one but has been deprecated".
But the "impact" might be lesser than showing we're able to implement a first class construct from the language.

[josevalim]

So let’s go with &&?

[sabiwara]

I went with a different approach and just changed the conclusion using if: 224b861

The problem with && is to find a good name: if we call this macro_and it would be confusing, and and is not a simple one-liner that can be implemented with an if.