Remove default migration group

References #77

config/default/migrate_plus.migration_group.default.yml

@@ -1,10 +0,0 @@
-uuid: 9d4454c4-6fff-4814-bfa1-8f152551619f
-langcode: en
-status: true
-dependencies: {  }
-id: default
-label: Default
-description: 'A container for any migrations not explicitly assigned to a group.'
-source_type: null
-module: null
-shared_configuration: null