Remove ForeignKey relations involving CustomInventoryScript

Add migration to carry out corresponding schema change

Add data migration to delete inventory sources
  which are the custom type

Split migration into two files
  data migration touches same rows as schema migration

awx/main/models/inventory.py

@@ -821,7 +821,6 @@ class InventorySourceOptions(BaseModel):
         ('openstack', _('OpenStack')),
         ('rhv', _('Red Hat Virtualization')),
         ('tower', _('Ansible Tower')),
-        ('custom', _('Custom Script')),
     ]
 
     # From the options of the Django management base command
@@ -845,14 +844,6 @@ class InventorySourceOptions(BaseModel):
         blank=True,
         default='',
     )
-    # TODO: migrate away
-    source_script = models.ForeignKey(
-        'CustomInventoryScript',
-        null=True,
-        default=None,
-        blank=True,
-        on_delete=models.SET_NULL,
-    )
     source_vars = models.TextField(
         blank=True,
         default='',
@@ -1329,7 +1320,6 @@ class InventoryUpdate(UnifiedJob, InventorySourceOptions, JobNotificationMixin,
 class CustomInventoryScript(CommonModelNameNotUnique, ResourceMixin):
     class Meta:
         app_label = 'main'
-        unique_together = [('name', 'organization')]
         ordering = ('name',)
 
     script = prevent_search(
@@ -1339,21 +1329,6 @@ class CustomInventoryScript(CommonModelNameNotUnique, ResourceMixin):
             help_text=_('Inventory script contents'),
         )
     )
-    organization = models.ForeignKey(
-        'Organization',
-        related_name='custom_inventory_scripts',
-        help_text=_('Organization owning this inventory script'),
-        blank=False,
-        null=True,
-        on_delete=models.SET_NULL,
-    )
-
-    admin_role = ImplicitRoleField(
-        parent_role='organization.admin_role',
-    )
-    read_role = ImplicitRoleField(
-        parent_role=['organization.auditor_role', 'organization.member_role', 'admin_role'],
-    )
 
     def get_absolute_url(self, request=None):
         return reverse('api:inventory_script_detail', kwargs={'pk': self.pk}, request=request)