Only Syndic minions have .local; push .local cleanup down into Syndic class

************* Module salt.minion
E1101:1126,16:Minion.destroy: Instance of 'Minion' has no 'local' member
Merge pull request #6018 from cvrebert/syndic-local-pylint

Only Syndic minions have .local; push .local cleanup down into Syndic class