For :all bottle blocks in linuxbrew-core, we are going to remove the bottle
block completely before rebottling.
In linuxbrew-core, we use --keep-old to keep the macOS sha lines.
I am not sure why this exception was written initially, but it
now prevents bottling these formulae, because we have no bottle block
and use --keep-old.
I think it's safe to just skip this check, when there is no bottle block
the old_checksum can just directly return nil.
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