Remove recommendation for Adium

[conorsch]

See discussion at #240 (comment) In short, Adium is not a reliable solution for present-day OS X users. We should investigate other chat clients for replacement, but Adium should be removed from the document either way.

[rileyjshaw]

Somewhat related, we might want to mention Signal Desktop 👍

[conorsch]

@rileyjshaw Great idea. We're going republish the document in its current form in the near future, as step toward making this a living document, that can incorporate changes from the community more rapidly. Thanks for your input!