glx: Remove support for glXGetDriverConfig for old drivers

gallium implements __DRI_CONFIG_OPTIONS version 2.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20549>

src/glx/dri_common.c

@@ -749,20 +749,11 @@ get_driver_config(const char *driverName)
 
          if (ext->base.version >= 2)
             config = ext->getXml(driverName);
-         else
-            config = strdup(ext->xml);
 
          break;
       }
    }
 
-   if (!config) {
-      /* Fall back to the old method */
-      config = dlsym(handle, "__driConfigOptions");
-      if (config)
-         config = strdup(config);
-   }
-
    dlclose(handle);
 
    return config;