bpo-40268: Remove unused structmember.h includes (GH-19530)

If only offsetof() is needed: include stddef.h instead.

When structmember.h is used, add a comment explaining that
PyMemberDef is used.
diff --git a/Objects/bytearrayobject.c b/Objects/bytearrayobject.c
index 4d1ddec..f05a98a 100644
--- a/Objects/bytearrayobject.c
+++ b/Objects/bytearrayobject.c
@@ -2,11 +2,10 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
-#include "pycore_abstract.h"   // _PyIndex_Check()
+#include "pycore_abstract.h"      // _PyIndex_Check()
 #include "pycore_bytes_methods.h"
 #include "pycore_object.h"
 #include "pycore_pymem.h"
-#include "structmember.h"
 #include "bytesobject.h"
 #include "pystrhex.h"