remove redundant experiment (#88)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index c7126ca..fbe6079 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -7,8 +7,6 @@
     unused_import: error
     unused_local_variable: error
     dead_code: error
-  enable-experiment:
-    - non-nullable
 linter:
   rules:
     - annotate_overrides