[REF] mrp: remove indexes of mrp.production (date_x)

Remove index on fields `date_start` and `date_finished`. Both
has not used expected in the gantt view `view_production_gantt`
which not used anymore (and have no sense because it should use
`date_planned_start` and `date_planned_finished` instead.
Remove this old gantt view too.

task-2389380

addons/mrp/models/mrp_production.py

@@ -136,8 +136,8 @@ class MrpProduction(models.Model):
     date_deadline = fields.Datetime(
         'Deadline', copy=False, store=True, readonly=True, compute='_compute_date_deadline', inverse='_set_date_deadline',
         help="Informative date allowing to define when the manufacturing order should be processed at the latest to fulfill delivery on time.")
-    date_start = fields.Datetime('Start Date', copy=False, index=True, readonly=True)
-    date_finished = fields.Datetime('End Date', copy=False, index=True, readonly=True)
+    date_start = fields.Datetime('Start Date', copy=False, readonly=True, help="Date of the WO")
+    date_finished = fields.Datetime('End Date', copy=False, readonly=True, help="Date when the MO has been close")
     bom_id = fields.Many2one(
         'mrp.bom', 'Bill of Material',
         readonly=True, states={'draft': [('readonly', False)]},

addons/mrp/views/mrp_production_views.xml

@@ -425,15 +425,6 @@
             </field>
         </record>
 
-        <record id="view_production_gantt" model="ir.ui.view">
-            <field name="name">mrp.production.gantt</field>
-            <field name="model">mrp.production</field>
-            <field name="arch" type="xml">
-                <gantt date_stop="date_finished" date_start="date_start" string="Productions" create="0">
-                </gantt>
-            </field>
-        </record>
-
         <record id="view_production_pivot" model="ir.ui.view">
             <field name="name">mrp.production.pivot</field>
             <field name="model">mrp.production</field>