Element-R: Remove "Upgrade your encryption" flow

[richvdh]

If we detect that you have key backup, but no 4S, we pop up a toast prompting you to "Upgrade your encryption".

This is broken in several ways:

• We also show the toast in the situation that you have 4S, but the backup key is not stored in 4S [at least, not under the current 4S key]: see Key Backup Failing to proceed. #27100 (comment). This is not a situation we can "upgrade" from.
• Generally, I don't think this is tested and I'm not convinced it works at all. Certainly it doesn't report any failures correctly (see Migrating key backup to 4s fails #27383).

Frankly, I don't think this is a flow we should be supporting at all. We should remove the "upgrade your encryption" flow. However, if it's something that we want to keep, then it needs tests and we need to fix the bugs above.

[richvdh]

After discussion with the team today: we're agreed that this flow should be removed altogether. When logging into an account that has key backup but the keys for that backup are not available in 4S, the only option will be to create a new key backup.