Remove Convert from custom inclusion fee

[lisa-parity]

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[lisa-parity]

I'm not sure if this change is correct because the referenced PR (and others) seem to be about trait WeightToFee and subtrait WeightToFeePolynomial (I didn't find a reference for a Convert trait). The only place I've seen Convert was in this release analysis post: https://forum.parity.io/t/polkadot-release-analysis/1419/2#span-stylefont-size24pxcolor-34b3f1font-weight-600text-decoration-underlineadd-constfeemultiplier-to-the-transaction-payment-palletspanhttpsgithubcomparitytechsubstratepull12222-22