Drop ModuleSelfAnnotation arm absent from pinned v4.0.2 AST

The generated Node enum is built from vendor/rbs/config.yml, which is
synced from the pinned v4.0.2 tag. v4.0.2 has no ModuleSelfAnnotation
node, so node_kind referenced a non-existent variant and failed to
compile under cargo clippy / cargo test.

https://claude.ai/code/session_01Bb4HnSKGf3PMGo1F8LUpz4

Author: claude

rust/ruby-rbs/src/ast/convert.rs

@@ -1212,7 +1212,6 @@ fn node_kind(node: &Node<'_>) -> &'static str {
         Node::InstanceVariableAnnotation(_) => "InstanceVariableAnnotation",
         Node::MethodTypesAnnotation(_) => "MethodTypesAnnotation",
         Node::ModuleAliasAnnotation(_) => "ModuleAliasAnnotation",
-        Node::ModuleSelfAnnotation(_) => "ModuleSelfAnnotation",
         Node::NodeTypeAssertion(_) => "NodeTypeAssertion",
         Node::ParamTypeAnnotation(_) => "ParamTypeAnnotation",
         Node::ReturnTypeAnnotation(_) => "ReturnTypeAnnotation",