Proto: simplify activity rules
The current activity rules (specifying when a delegate is de-activated) are not very easy to understand, and the code is hard to follow.
Furthermore, with Tenderbake we may want that non-participation leads more quickly to deactivation, in order to reduce the risk of not reaching the required quorum due to inactive delegates. Finally, the activity rules are defined in terms of the constant preserved_cycles
. It's not clear that this dependency is still needed.