Fix compile error with Z_DUAL_ENDSTOPS

Marlin/pins_RAMPS_13.h

 #define Y2_DIR_PIN         34
 #define Y2_ENABLE_PIN      30
 
+#undef Z2_STEP_PIN
+#undef Z2_DIR_PIN
+#undef Z2_ENABLE_PIN
+
 #define Z2_STEP_PIN        36
 #define Z2_DIR_PIN         34
 #define Z2_ENABLE_PIN      30

Marlin/stepper.cpp

   static bool old_z_max_endstop = false;
 #endif
 #ifdef Z_DUAL_ENDSTOPS
-  #if HAS_Z2_MIN
+  // #if HAS_Z2_MIN
     static bool old_z2_min_endstop = false;
-  #endif
-  #if HAS_Z2_MAX
+  // #endif
+  // #if HAS_Z2_MAX
     static bool old_z2_max_endstop = false;
-  #endif
+  // #endif
 #endif
 
 #ifdef Z_PROBE_ENDSTOP // No need to check for valid pin, SanityCheck.h already does this.
-static bool old_z_probe_endstop = false;
+  static bool old_z_probe_endstop = false;
 #endif
 
 static bool check_endstops = true;