Nomination pools: Fix payout destination in permissionless unbond (#3110)

This fixes a bug in nomination pools that msitakenly claimed the
rewards, upon pool destruction, into the caller of `unbond` and not the
actual member.

More description and a PA coming soon. Opening this for now to expediate
backport.

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Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>