Revert "Invalid default scenarios"
Created by: dverdin
Reverts sympa-community/sympa#528 A lot of problems with this PR. Sorry I did not see them before. The problem is: listmasters can't define default scenarios anymore. the only way to do so is in ListDef.pm Consequently : all instances having defined default scenarios will have their customizations ignored when upgrading. This PR is clearly a regression and should be removed.