Rethink revoke_membership call

MR !215 (merged) adds a new identity status "Revoked" that removes all memberships. While pallet-membership is still used for multiple instances (main wot membership and smith membership), it could have a meaning to keep it to express "I want to leave smith wot". But a refac of smith membership could make this call obsolete and redundant with call "revoke_identity".

if smith membership instance is removed, remove revoke_membership call entirely
if smith membership instance is kept, maybe delete the smith certs received after revoke_membership is called

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