Remove irrelevant mentions of Tails Installer, post USB images

In https://tails.boum.org/doc/first_steps/persistence, it is said twice that one need a device installed with Tails installer in order to set up a persistent volume:

It is only possible to create a persistent volume if the USB stick was installed using Tails Installer.

Now that it is possible to install IMG images with Etcher, that’s not true anymore and should be fixed

Feature Branch: doc/16548-remove-tails-installer-mentions

Original created by @goupille on 16548 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information