remove redundant experiment

pkgs/stream_channel/analysis_options.yaml

diff --git a/pkgs/stream_channel/analysis_options.yaml b/pkgs/stream_channel/analysis_options.yaml
index 8a712df..b36c874 100644
--- a/pkgs/stream_channel/analysis_options.yaml
+++ b/pkgs/stream_channel/analysis_options.yaml
@@ -8,8 +8,6 @@
     unused_element: error
     unused_local_variable: error
     dead_code: error
-  enable-experiment:
-    - non-nullable
 
 linter:
   rules: