libcrc32c: remove crc32c_impl

This was only ever used by btrfs, and the usage just went away.
This effectively reverts df91f56a

 ("libcrc32c: Add crc32c_impl
function").

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>