Use reprepro unreferencesnapshot instead of _removereferences

reprepro 5.1.0-1 now has nice way to delete all references to a given snapshot. We should switch our code and doc to using it, instead of reprepro dumpreferences | […] | reprepro _removereferences that we use (at least) in:

tails-delete-expired-apt-snapshots in puppet-tails
wiki/src/contribute/APT_repository/time-based_snapshots.mdwn

Related issues

Related to #11912 (closed)

Original created by @intrigeri on 12096 (Redmine)

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