Remove unused ReaderObject_Check macro (#20614)

diff --git a/Modules/_csv.c b/Modules/_csv.c
index 3a52632..f33733a 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -112,8 +112,6 @@
 
 static PyTypeObject Reader_Type;
 
-#define ReaderObject_Check(v)   Py_IS_TYPE(v, &Reader_Type)
-
 typedef struct {
     PyObject_HEAD