software/erp5/test: remove PYTHONPATH hack

we no longer run tests with `python setup.py test`, so this hack is no
longer necessary.

See merge request !871
13 jobs for master in 0 seconds
Status Job ID Name Coverage
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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

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passed SlapOS.SlapRunner.ResilienceTest-Master

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passed SlapOS.SlapRunner.ResilienceTest-Master

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failed SlapOS.SlapRunner.ResilienceTest-Master.ERP5

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failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python2

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failed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

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passed SlapOS.SoftwareReleases.IntegrationTest-Master.Python3

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