erofs-utils: lib: drop prefix_sha256 digests

People care more about speed when building.  Later, we could also
try to use file-backed memory to avoid memory pressure.

As a result, it could decrease time by 50% (0m58.255s -> 0m29.120s).

Link: https://lore.kernel.org/r/20230922183055.1583756-3-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
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