model: no longer use DiskBackedContent

Everything is a Content now. So we no longer needs this complexity.

Jenkins job DLDCVS/gitlab-builds #38 failed .
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mentioned in merge request swh-model!347 (merged)

This needs this MR to be merged swh-model!347 (merged), and an actual 6.13 version of swh-model to released.

marked this merge request as draft

marked this merge request as ready

@jenkins retry build

Jenkins job DLDCVS/gitlab-builds #39 succeeded .
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approved this merge request

merged