Remove separate ReferenceContext enum (#4631)

crates/ruff_python_semantic/src/context.rs

@@ -0,0 +1,7 @@
+#[derive(Debug, Copy, Clone, is_macro::Is)]
+pub enum ExecutionContext {
+    /// The reference occurs in a runtime context.
+    Runtime,
+    /// The reference occurs in a typing-only context.
+    Typing,
+}