Problem
---------
In 15, taxes are hidden from hr.expense when the expense category has a
cost. They can however be configured and added. The current behaviour is
counter intuitive.

Objective
---------
Don't hide taxes when product has cost in v15.

Solution
---------
Remove the `hidden` attribute in the expense xml form as well as the
`groups` attribute.

task-3491868

closes odoo/odoo#134147

Signed-off-by: Quentin De Paoli <qdp@odoo.com>