Remove outdated lints (dart-lang/pubspec_parse#119)

pkgs/pubspec_parse/analysis_options.yaml

diff --git a/pkgs/pubspec_parse/analysis_options.yaml b/pkgs/pubspec_parse/analysis_options.yaml
index 7da0e89..c209999 100644
--- a/pkgs/pubspec_parse/analysis_options.yaml
+++ b/pkgs/pubspec_parse/analysis_options.yaml
@@ -12,8 +12,6 @@
     - 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