Version 16.0 removed the price different account following this pull
request https://github.com/odoo/odoo/pull/99411

This decision has been made because it was only used anymore by
standard cost method and real time valuation. We thought that standard
was not a valid accounting method and we didn't want to maintain
code for it. But:
- Standard could be valid if you manualy complete the accounting entries
by yourself (e.g. employees/machines cost in mrp).
- It's also accepted as legal inventory valuation if you record the
  difference between invoice and standard price + regular update of
the standard price to match the reality
- If you want to have an estimation of your cogs (benefits and loss)
during an accounting period. People just do a manual correction at
the end but they have a real time reporting on the situation.

Since delivery/receipt interim accounts already exists we could say
that user could simply reconcile them by himself. It could be but
it would be a pain to know what part of the account is already invoiced
and need a correction vs the part not yet invoiced.

This commit remove the fix module added in 16.0 and put the fields back
in purchase