[modular][cleanup] Remove unused UserLoginParams.session_shell_config.

TESTED: paved
Change-Id: I67bd5ce6ba6c30d9af4d553b822df8bca6417621
diff --git a/public/fidl/fuchsia.modular/basemgr/user_provider.fidl b/public/fidl/fuchsia.modular/basemgr/user_provider.fidl
index 9025918..45aeca7 100644
--- a/public/fidl/fuchsia.modular/basemgr/user_provider.fidl
+++ b/public/fidl/fuchsia.modular/basemgr/user_provider.fidl
@@ -59,10 +59,6 @@
 
     // If login fails, |user_controller| is closed.
     request<UserController> user_controller;
-
-    // Configuration of the session shell to run. It can be NULL which means that
-    // this configuration is picked up from command-line arguments to basemgr.
-    AppConfig? session_shell_config;
 };
 
 // Provided by the |Basemgr| to the |BaseShell| when it authenticates a