Withdraw Assets Before Checking Out in `OnReapIdentity` impl (#2552)

Follow up to fix a bug from #1814 discovered in XCM emulator testing.

I mistakenly thought that checking out an asset would withdraw it from
the sender. This actually withdraws the asset before checking out.

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>