Don't reject audio-only VOD assets

[thomshutt]

This is now supported in Catalyst API with livepeer/catalyst-api#843

[mjh1]

This probe call is no longer used by the upload task (#212), however I did notice it's still in use by 'transcode' (not transcode API) and 'import' tasks. @victorges could you remind me are these tasks in use anymore? If not would it be safe to delete them?

[victorges]

@mjh1 Those are the legacy flows for VOD. Import is equivalent to the Upload today, while transcode was a "mutation" of existing assets like to ask for a different profile for the master rendition or something like that. I believe it is not used anywhere, we don't document it nor expose on SDKs at least, and I guess they might even be completely broken by now. You can make sure by looking for tasks of type import or transcode on the DB, but I think it should be mostly safe to delete yeah!