remove redundant experiment (#44)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index c03796b..4a774a7 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -6,8 +6,6 @@
   errors:
     annotate_overrides: ignore
     prefer_single_quotes: ignore
-  enable-experiment:
-    - non-nullable
 
 linter:
   rules: