CRM leads deletion

After a short conversation with fp, al, odo, abo: it seems that it would be nice for usability's sake to remove restrictions on leads deletion, in the CRM module.
This condition on leads' state was introduced several years ago by fp to avoid unfortunate misclicks. It has been followed by another one bypassing the first one in case where the sales team had the "allow_unlink" flag set.

Since the "Delete" button has been thrown away in the "More" menu, it shouldn't get misclicked anymore, preventing damages in the CRM pipe. This will simplify access rights for users: if you want to delete a lead, you can, whatever its state, whatever its sales team.

Access restriction is still possible though by creating a record rule with a domain on the lead.state.