Remove redundant part of MembershipPolicy
Remove redundant part of MembershipPolicy

Reasoning according to @tf: The first part of the conditional appears
to be redundant. `add_membership_on?` (which `destroy_membership_on?`
delegates to) will always be true anyway if the first part of the
conditional is true.

Modularfield/pageflow,codevise/pageflow,tf/pageflow,Modularfield/pageflow,tf/pageflow,Modularfield/pageflow,codevise/pageflow,tf/pageflow,Modularfield/pageflow,tf/pageflow,codevise/pageflow,codevise/pageflow