pallet-membership weights (#3324)

## Summary
* use benchamarked weights instead of hardcoded ones for
`pallet-membership`
* rename benchmark to match extrinsic name
* remove unnecessary dependency from `clear_prime`

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Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>