switch-branch: remove system reinstall on switch

With introduction of link signatures for packages updater can now detect changes in link dependencies and reinstall package without relying on package bumps. Thanks to that we no longer have to reinstall all packages when switching between unsafe branches as link signature is going to change and updater is going to reinstall affected packages without need to reinstall all packages.

This also allows easy to simulate update between no just stable branches but also between unstable ones (usable for testing of updates to future major or minor release).

Resolves https://gitlab.nic.cz/turris/turris-os-packages/-/issues/638