[PERF] account_external_tax: avoid redundant tax clearing

Avoid unnecessarily clearing `tax_ids` before setting the externally
computed taxes, as the field is immediately replaced with the new tax
values.

Performance testing was performed using an invoice containing 200 lines
with externally computed AvaTax taxes.

Performance testing:

| Metric                         | Before | After   |
|--------------------------------|--------|---------|
| `_set_external_taxes()`        | ~1:13  | ~43.77s |
| Total external tax calculation | ~1:16  | ~46.66s |

This reduces the execution time of `_set_external_taxes()` by
approximately 40% for a 200-line invoice, while preserving the existing
functional flow.

The optimization is not specific to AvaTax and benefits the common
`account.external.tax.mixin` flow when setting externally computed taxes
on multiple lines.

Profiler comparison:

Before:
<img width="1548" height="442" alt="image" src="https://github.com/user-attachments/assets/a5f2553c-a926-4324-88b2-743ed9214bf5" />


After:
<img width="1429" height="440" alt="image" src="https://github.com/user-attachments/assets/c9c9c673-4654-4e11-b5f2-ffa4ecc87038" />


**opw-6472692**

Forward-Port-Of: odoo/enterprise#130982