Remove removed lints (dart-lang/web_socket_channel#299)

pkgs/web_socket_channel/analysis_options.yaml

diff --git a/pkgs/web_socket_channel/analysis_options.yaml b/pkgs/web_socket_channel/analysis_options.yaml
index b7d68c9..fbdbd4e 100644
--- a/pkgs/web_socket_channel/analysis_options.yaml
+++ b/pkgs/web_socket_channel/analysis_options.yaml
@@ -10,8 +10,6 @@
   - avoid_classes_with_only_static_members
   - avoid_private_typedef_functions
   - avoid_redundant_argument_values
-  - avoid_returning_null
-  - avoid_returning_null_for_future
   - avoid_returning_this
   - avoid_unused_constructor_parameters
   - avoid_void_async