[REF] mrp: remove index on state (mrp.unbuild)

The state of `mrp.unbuild` is a boolean value (`draft` or `done`) which
is already quite useless in the most of the case. Also this state is
never used explicitly in the code (in a search), then it seems legit to
remove it.

task-2389380

addons/mrp/models/mrp_unbuild.py

@@ -70,7 +70,7 @@ class MrpUnbuild(models.Model):
         string='Processed Disassembly Lines')
     state = fields.Selection([
         ('draft', 'Draft'),
-        ('done', 'Done')], string='Status', default='draft', index=True)
+        ('done', 'Done')], string='Status', default='draft')
     allowed_mo_ids = fields.One2many('mrp.production', compute='_compute_allowed_mo_ids')
 
     @api.depends('company_id', 'product_id')