remove redundant experiment (#22)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 71546a6..007066a 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -8,5 +8,3 @@
     annotate_overrides: ignore
     prefer_single_quotes: ignore
     use_function_type_syntax_for_parameters: ignore
-  enable-experiment:
-    - non-nullable