Remove crispa from install page

[jgoerzen]

The crispa android app isn't supporting 0.4 -- at least not the version on f-droid or on the page linked to from the install page. Over at yggdrasil-network/crispa-android#58 I reported this, and also included a link to a fork for 0.4 that was mentioned on the Matrix channel.

In any case, it seems to be presenting incoming users with something that seems to not work, and is causing confusion.