[FIX] purchase: remove wrongly set `oldname` attribute

In version 8.0, there was a stored field `minimum_planned_date`.
In version 9.0, this field was renamed `date_planned` and loose it
store attribute.
In version 10.0. the field is stored again.

Keeping this `oldname` attribute will lead to reuse old data from
version 8.0 when migrating the database to version 10.0.

Closes #17631
opw-744097