Preserve enrollment when deleting enrollment pkg

[coveralls]

Pull Request Test Coverage Report for Build 9235321869

Details

• 3 of 3 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.002%) to 78.678%

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Files with Coverage Reduction	New Missed Lines	%
zentral/contrib/monolith/models.py	1	89.35%
<!--	Total:	1		-->

Totals
Change from base Build 9180399256:	-0.002%
Covered Lines:	35770
Relevant Lines:	45464

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💛 - Coveralls