refactor(SellMembershipAuthority): remove outdated logic related to refunds

[0xble]

NOTE: This does not require a re-deploy of SellMembershipAuthority. It behaves the same way but updates some comments and outdated and unnecessary logic when returning contribution amount to emit.

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[github-actions]

Gas Report:

[0xble]

TODO: Fix tests

[arr00]

This just went through an audit and changes provide no material benefits. I don't think this should be prioritized now.

[0xble]

This just went through an audit and changes provide no material benefits. I don't think this should be prioritized now.

I actually didn't imagine we'd deploy this until the next significant change involving SellMembershipAuthority so yeah I kinda agree. I am concerned though that not updating in our remote repo will confuse people reading that code.