Steps to reproduce
------------------
* install l10n_de
* switch to a german company
* create an invoice
* add a line such that the account's default tax is different from the
  tax set on the line (ex, account: `8400 Erlöse 19% USt` and
  tax: `7% Umsatzsteuer`)

Attempt to confirm the invoice, you should see that you can't. The
system requires the account's default tax to be the same as the tax set
on the line. This should not be the case. It's a practice in Germany to
link a tax to an account like this, but it's not a legal requirement.

opw-3324323

closes odoo/odoo#127238

Signed-off-by: Habib Ayob (ayh) <ayh@odoo.com>