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).