Revert "main: Make sure that --mode parameter is valid"

This reverts commit a7a46bbe1c.

Conflicts:

	js/ui/sessionMode.js

src/shell-global.c

@@ -585,21 +585,6 @@ shell_global_set_stage_input_mode (ShellGlobal         *global,
     }
 }
 
-gboolean
-_shell_global_has_valid_session_mode (ShellGlobal *global)
-{
-  char *script;
-  int status;
-
-  script = g_strdup_printf ("imports.ui.environment.init();"
-                            "imports.ui.sessionMode.modeExists('%s') || 1;",
-                            global->session_mode);
-  gjs_context_eval (global->js_context, script, -1, "<sessionMode.js>", &status, NULL);
-  g_free (script);
-
-  return status == 0;
-}
-
 /**
  * shell_global_set_cursor:
  * @global: A #ShellGlobal