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/constants.py

@@ -7,7 +7,6 @@ from django.utils.translation import ugettext_lazy as _
 
 __all__ = [
     'CLOUD_PROVIDERS',
-    'SCHEDULEABLE_PROVIDERS',
     'PRIVILEGE_ESCALATION_METHODS',
     'ANSI_SGR_PATTERN',
     'CAN_CANCEL',
@@ -16,10 +15,6 @@ __all__ = [
 ]
 
 CLOUD_PROVIDERS = ('azure_rm', 'ec2', 'gce', 'vmware', 'openstack', 'rhv', 'satellite6', 'tower')
-SCHEDULEABLE_PROVIDERS = CLOUD_PROVIDERS + (
-    'custom',
-    'scm',
-)
 PRIVILEGE_ESCALATION_METHODS = [
     ('sudo', _('Sudo')),
     ('su', _('Su')),