I know it's a bit of a pain, but you can't (or at least Microsoft says so) manually remove Exchange 2007.
The supported way is to uninstall it properly. Since you can't because the server no longer exists then you can install a temporary server with same name and run setup /m:RecoverServer to recover the crashed server and once it is recovered then uninstall it properly.
If you try this and it fails horribly (it should work), then you can try to manually remove the exchange objects from the schema using adsiedit.msc . Be very careful if you ultimately decide to follow this path as you can completely mess up your AD and destroy your domain if you do it wrong.
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