package.utils: Drop unneeded hashes from download computation

This should fix the current frequent issue on the debian loader [1]

• loader retrieve badly the length information from the content-length header
• We check for download size with that incorrect data
• so download call function fails because it checks the local size and it's difference

all of this for the sha1_git computation which needs the length. But we do not really need that hash (nor blake).

So simplifying by removing will actually fix [1]

• [1] https://sentry.softwareheritage.org/share/issue/49d464bee7b24ad080598b32cd3eb9a8/

• [2] https://sentry.softwareheritage.org/organizations/swh/issues/?project=9&query=is%3Aunresolved&sort=freq&statsPeriod=14d

Test Plan

tox

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Migrated from D2486 (view on Phabricator)