remove redundant experiment

analysis_options.yaml

diff --git a/analysis_options.yaml b/analysis_options.yaml
index da905ca..76c449a 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -4,5 +4,3 @@
     implicit-casts: false
   language:
     strict-raw-types: true
-  enable-experiment:
-    - non-nullable