`expire_memberships` benchmark fail

The expire_memberships benchmark has been failing since !215 (merged) without triggering error (because it's a hook): failed to remove member: DispatchErrorWithPostInfo { post_info: PostDispatchInfo { actual_weight: None, pays_fee: Pays::Yes }, error: Module(ModuleError { index: 10, error: [6, 0, 0, 0], message: Some("MemberNotFound") }) } and Logic error: fail to purge session keys in do_remove_member(): Module(ModuleError { index: 14, error: [3, 0, 0, 0], message: Some("NoKeys") })

To do:

• benchmark with maximum validators available and perform arithmetic.