[REM] core: remove `test_documentation_examples` module

The only purpose of the module `test_documentation_examples` is to test
a few of the code examples shown in the developer documentation.

As 6709e1a9794 removes the documentation sources from the repository,
the test module is removed as well.

DOC PR: https://github.com/odoo/documentation-user/pull/945



task-2352371

closes odoo/odoo#70174

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>
Co-authored-by: Victor Feyens <vfe@odoo.com>
Co-authored-by: Antoine Vandevenne <anv@odoo.com>