tests: Remove no longer needed pytest custom marker named db

This was used at the time we were building debian packages for swh components but we no longer do that.

Jenkins job DCORE/gitlab-builds #307 succeeded in 55 sec.
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added 6 commits

• 137fe291...955c001f - 5 commits from branch swh/devel:master
• ee7109b7 - tests: Remove no longer needed pytest custom marker named db

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merged

Jenkins job DCORE/gitlab-builds #323 succeeded in 1 min 11 sec.
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