[FIX] util/models: Prevent removal manual relation table

There are two model ``A`` and ``B`` and there is
``x`` many2many field in ``A`` model which is manual
and relation with ``B``. Now, ``B`` model is removed.
So, For that relation table is droped but the field will there
and in relation model ``_unknown`` will be set
https://github.com/odoo/upgrade-util/blob/25cb37389b527f442acfbb704e07769d69b11b9f/src/util/models.py#L242.

As the field belongs to ``A``. So, table will be recreated
from here as it checks only table is there or not
https://github.com/odoo/odoo/blob/54f10b56f577ad9ed5575bd396dba7d20d22fc2e/odoo/fields.py#L4977
and after that from here env variable it have in gone
and below menitoned traceback will raise. To Fix, Preventing
table for manual fields. So, field can be checked later on and to prevent
blocking.

```
Traceback (most recent call last):
  File "/home/odoo/src/odoo/19.0/odoo/service/server.py", line 1509, in preload_registries
    registry = Registry.new(dbname, update_module=update_module, install_modules=config['init'], upgrade_modules=config['update'], reinit_modules=config['reinit'])
  File "/home/odoo/src/odoo/19.0/odoo/tools/func.py", line 88, in locked
    return func(inst, *args, **kwargs)
  File "/home/odoo/src/odoo/19.0/odoo/orm/registry.py", line 185, in new
    load_modules(
  File "/home/odoo/src/odoo/19.0/odoo/modules/loading.py", line 580, in load_modules
    model._register_hook()
  File "/tmp/tmpkw62chtg/migrations/base/0.0.0/pre-models-ir_model_relation.py", line 49, in _register_hook
    raise util.MigrationError("The following m2m relations have respawn:\n%s" % back_m2m)
odoo.upgrade.util.exceptions.UpgradeError: The following m2m relations have respawn:
 - x_res_partner_uom_category_rel via uom.category
```
upg-3191893
opw-5175564

Author: sagu-odoo

src/util/models.py

@@ -193,6 +193,35 @@ def remove_model(cr, model, drop_table=True, ignore_m2m=()):
         if ignore_m2m != "*":
             tables = get_m2m_tables(cr, table_of_model(cr, model))
             ignore = set(ignore_m2m)
+            # manual fields M2M handle should be explicitly
+            if tables:
+                cr.execute(
+                    """
+                    SELECT name, model
+                      FROM ir_model_fields
+                     WHERE relation_table IN %s
+                       AND state = 'manual'
+                        """,
+                    [tuple(tables)],
+                )
+
+                if cr.rowcount:
+                    affected_fields = cr.fetchall()
+                    for field, field_model in affected_fields:
+                        remove_field(cr, field, field_model)
+                    add_to_migration_reports(
+                        message="""
+                              The following Many2Many fields have been deprecated because their related %s (%s)
+                              model is deprecated:%s
+                            """
+                        % (
+                            model,
+                            mod_label,
+                            "\n".join("{}".format(field) for field, _ in affected_fields),
+                        ),
+                        category="Removed Fields",
+                    )
+
             for table_name in tables:
                 if table_name in ignore:
                     _logger.info("remove_model(%r): m2m table %r explicitly ignored", model, table_name)