[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
removing M2m field and added into migration report

```
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_model, field, drop_column=False)
+                    add_to_migration_reports(
+                        message="""
+                              The following Many2Many fields have been deprecated because their related %s (%s)
+                              model is deprecated: \n%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)