variable.c: remove unused macros

Co-authored-by: Gemini <gemini@google.com>

src/variable.c

@@ -19,9 +19,7 @@ typedef struct iv_tbl {
   mrb_value *ptr;
 } iv_tbl;
 
-#define IV_EMPTY 0
-#define IV_DELETED (1UL<<31)
-#define IV_KEY_P(k) (((k)&~((uint32_t)IV_DELETED))!=0)
+
 
 /* Creates the instance variable table. */
 static iv_tbl*