Drop Riot from IM and Email Alternatives categories

We should not recommend software that is not end-to-end encrypted by default for instant messaging (or any private one-to-one messaging, for that matter). Riot might still be fine as an alternative to IRC or Slack.

This commit also removes duplicated non-localized Riot entry from all locales except en.

See: https://github.com/nylira/prism-break/pull/1178#issuecomment-379494346

We already have Signal, Ricochet etc that provide strong security on networked endpoints. Yet Prism-Break takes time to feature "experimental" apps like Riot that do not provide end-to-end encryption by default. I mean, Facebook Messenger, Google Allo and Telegram all sit in the same boat, yet the site shuns away from those.

There's also #1936 (closed).