xcm: avoid subtracting delivery fees twice (#7201)

At the moment, the delivery fees are being subtracted twice if the
`jit_withdraw` mode is enabled, resulting in assets being trapped. [This
is an example of
it](https://dev.papi.how/explorer/0x4a89cc6d157c28bdf42a1846ea33fffaf1f33d7091cefdd22a53075c73c5886d#networkId=paseo_asset_hub&endpoint=light-client).
This does not happen in 2412 anymore after #4834.

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Co-authored-by: Adrian Catangiu <adrian@parity.io>
Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: command-bot <>