[FIX] delivery: hide useless length UoM

The dimensions UoM of the carrier packaging is useless: it is not the
one used in the requests and there is not any conversion

To reproduce the issue
(Need delivery_fedex. Use demo data. Enable debug mode)
1. In Shipping Methods, edit 'Fedex US':
    - Package Weight Unit: KG
    - Debug Requests: True
2. Edit its Fedex Package Type:
    - Height: 1m
    - Width: 1m
    - Length: 1m
    - Package Code: YOUR_PACKAGING
3. Create a SO with a US partner and one product
4. On the SO
    - Add Shipping
    - Select 'Fedex US'
    - Click on 'Get Rate'
5. In Logging, open the request sent

Error: the dimensions of the packaging are expressed with "CM" but they
are not converted:
```xml
<ns0:Dimensions>
	<ns0:Length>1</ns0:Length>
	<ns0:Width>1</ns0:Width>
	<ns0:Height>1</ns0:Height>
	<ns0:Units>CM</ns0:Units>
</ns0:Dimensions>
```

When encoding the packaging in the request, the UoM defined on the
packaging (meter) is ignored. Instead, we define a length UoM depending
on the UoM of the weight and we don't convert any dimension:
https://github.com/odoo/enterprise/blob/e4fd13a0e073b9855864b7fabc26bc05b9a5fd19/delivery_fedex/models/fedex_request.py#L172-L178
There is even a `TODO` about the issue.

It will be the same with the other carriers. For instance, with UPS:
https://github.com/odoo/enterprise/blob/c9899f7860cd19c86f215d6dcea89f73b51433f3/delivery_ups/models/ups_request.py#L98-L102


We use the UoM of `ups_package_dimension_unit` and do not convert the
dimensions.

We can't implement the dimensions conversion on stable versions, since
it will break all existing packagings. So, as temporary solution, we can
simply hide the useless UoM defined on the packagings.

OPW-3053048

closes odoo/odoo#106329

X-original-commit: 3bc2f594395a08515ed362e3d9a816e7cc18c842
Related: odoo/enterprise#34301
Signed-off-by: Arnold Moyaux (arm) <arm@odoo.com>