erp5_pdm: fix dead proxy field

follow up https://lab.nexedi.com/nexedi/erp5/-/merge_requests/2237, /cc @tsoulard

FAIL: test_deadProxyFields (CodingStyleTest.CodingStyleTest erp5_pdm.test_deadProxyFields)
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Traceback (most recent call last):
  File "/srv/slapgrid/slappart75/t/ekc/soft/ff7044a58df3c3cba08a0139db53e32c/parts/erp5/product/ERP5/tests/testXHTML.py", line 131, in test_deadProxyFields
    self.fail(message)
AssertionError:
Dead proxy field list('View', 'Transformation_view/my_effective_date', 'Base_viewTradeFieldLibrary', 'my_view_mode_effective_date')
	('View', 'Transformation_view/my_expiration_date', 'Base_viewTradeFieldLibrary', 'my_view_mode_expiration_date')