Missing purge_subnets parameter on function call

purge_subnets|routes were introduced recently.
ensure_subnet_associations now takes purge_subnets as a parameter.  A
call to this function was missed when introducing this feature.  With
out, results in a "got 5 expected 6" error.

(cherry picked from commit cd24bbbc9c)

lib/ansible/modules/cloud/amazon/ec2_vpc_route_table.py

@@ -464,6 +464,7 @@ def ensure_route_table_absent(connection, module):
     route_table_id = module.params.get('route_table_id')
     tags = module.params.get('tags')
     vpc_id = module.params.get('vpc_id')
+    purge_subnets = module.params.get('purge_subnets')
 
     if lookup == 'tag':
         if tags is not None:
@@ -485,7 +486,7 @@ def ensure_route_table_absent(connection, module):
         return {'changed': False}
 
     # disassociate subnets before deleting route table
-    ensure_subnet_associations(connection, vpc_id, route_table, [], module.check_mode)
+    ensure_subnet_associations(connection, vpc_id, route_table, [], module.check_mode, purge_subnets)
     try:
         connection.delete_route_table(route_table.id, dry_run=module.check_mode)
     except EC2ResponseError as e: