Remove useless/broken reexport of hashes

[Kixunil]

No types from hashes are used in the API (this is correct) however the reexport is still there. This wouldn't be that terrible (just a noise) if ranged version wasn't used. However with ranged versions, if someone depends on the reexport it can have a random dependency breaking randomly. #722 suggests to remove the ranged version but we might want it back again later and it's still noise to reexport.

[apoelstra]

ACK from me (assuming we have actually eliminated all types from the API). I believe you are correct that this will eliminate all breakage related to the ranged dep.

[Kixunil]

assuming we have actually eliminated all types from the API

Yes, I've checked.