Clean up unused grafanalib based dashboards

In effect, this reverts commit [1] "grafana: Auto-import grafanalib dashboards"

• [1] 17b2b304.

Related to T3343

Test Plan

bin/octocatalog-diff --octocatalog-diff-args --no-truncate-details --to staging pergamon
Found host pergamon.softwareheritage.org
...
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- File[/etc/grafana/provisioning/dashboards/10-grafanalib-dashboards.yaml]
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- Package[swh-grafanalib-dashboards]
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*** End octocatalog-diff on pergamon.softwareheritage.org

vagrant up pergamon (from scratch) no longer complains about it ^

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Migrated from D5777 (view on Phabricator)