[FIX] core: avoid modifying shared fields

[HydrionBurst]

when a field object is not _toplevel, it may be shared with multiple registries and should be readonly in any case

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:

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I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

[robodoo]

[rco-odoo]

@robodoo override=ci/security

The use of type in the code should not harm:

• it only happens when deleting some ir.model.fields record while uninstalling a module;
• regular users cannot change the name or model of ir.model.fields records, except for custom fields, and a field's new name cannot correspond to another field on the model (because of unicity constraint);
• the field object corresponding to the deleted record must be in the registry;
• the field's class is used to instanciate a new field that is equivalent to field except for prefetch=False.

[rco-odoo]

@robodoo r+

[rco-odoo]

@robodoo r-