They are an implementation detail of storage backends; and get_stats() will
stop returning the 'person' count in the next version of swh-storage-core.
Fixes builds [1] [2]
[1] https://jenkins.softwareheritage.org/job/DLDHG/job/tests/793
[2] https://jenkins.softwareheritage.org/job/DENV/job/tests/lastBuild/console
Similar to D3978