remove redundant experiment (#71)

diff --git a/analysis_options.yaml b/analysis_options.yaml
index 447aba8..b459a9b 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -2,8 +2,6 @@
 analyzer:
   strong-mode:
     implicit-casts: false
-  enable-experiment:
-    - non-nullable
 
 linter:
   rules: