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.

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Adrian Catangiu <adrian@parity.io>