Remove app_label variable since it's unused

fixes #5321

example-source/group_vars/all

@@ -4,7 +4,6 @@ pulp_source_dir: '/var/lib/pulp/devel/pulpcore'
 pulp_plugin_source_dir: "/var/lib/pulp/devel/pulpcore-plugin"
 pulp_install_plugins:
   pulp-file:
-    app_label: "file"
     source_dir: "/var/lib/pulp/devel/pulp_file"
 developer_user_home: /var/lib/pulp
 developer_user: pulp

example-use/group_vars/all

@@ -1,7 +1,6 @@
 ---
 pulp_default_admin_password: password
 pulp_install_plugins:
-  pulp-file:
-    app_label: "file"
+  pulp-file: {}
 pulp_settings:
   secret_key: secret

roles/pulp/README.md

@@ -18,9 +18,6 @@ Role Variables:
   Defaults to "{}", which will not install any plugins.
   * Dictionary Key: The pip installable plugin name. This is defined in each
   plugin's* `setup.py`. **Required**.
-  * `app_label`: Used to make migrations. This is defined in the
-  [PluginAppConfig](https://github.com/pulp/pulp_file/blob/76cc979e67fde128f78f3274697a4ea78f2269ec/pulp_file/app/__init__.py#L10)
-  of each plugin. **Required**.
   * `source_dir`: Optional. Absolute path to the plugin source code. If present,
   plugin will be installed from source in editable mode.
 * `pulp_install_api_service`: Whether to create systemd service files for