pallet-membership: Do not verify the `MembershipChanged` in bechmarks #13388

Job	Run time
check-dependency-rules	9s
check-fail-ci	13s
check-markdown	19s
check-workspace	23s
preflight / preflight	42s
preflight / ci-versions	1m 0s
fmt	1m 3s
check-umbrella	2m 15s
check-rust-feature-propagation	2m 3s
check-toml-format	55s
test-rust-features	2m 3s
All quick checks passed	1s
	11m 6s

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