[IMP] hr_expense: stop tracking the approver on expenses

The field is related towards the 'manager' field of the expense sheet,
which means that when the approver is suggested automatically based
on hr data, the expense records get written a value for this field
(which is not in the view) which gets logged as:
- None -> Mitchell Admin (Approved By)

which seems to suggest that:
- the expense has been approved (it has not, a report was created)
- Mitchell Admin has approved: it may not be him (as when approving,
the 'manager' field does not log who did the approval but who was
supposed to initially)

Disabling tracking for this field (which contained no actionable
information at best, and incorrect suggestions at worst) makes this
problem go away.

closes odoo/odoo#100241

Signed-off-by: Kevin Baptiste <kba@odoo.com>