pallet-membership: Do not verify the `MembershipChanged` in bechmarks (#6439)

There is no need to verify in the `pallet-membership` benchmark that the
`MemembershipChanged` implementation works as the pallet thinks it
should work. If you for example set it to `()`, `get_prime()` will
always return `None`.

TLDR: Remove the checks of `MembershipChanged` in the benchmarks to
support any kind of implementation.

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Co-authored-by: GitHub Action <[email protected]>
Co-authored-by: Adrian Catangiu <[email protected]>