Small reduction in M421 code

Marlin/Marlin_main.cpp

    * Use either 'M421 X<mm> Y<mm> Z<mm>' or 'M421 I<xindex> J<yindex> Z<mm>'
    */
   inline void gcode_M421() {
-    float x = 0, y = 0, z = 0;
-    int8_t px = 0, py = 0;
-    bool err = false, hasX, hasY, hasZ, hasI, hasJ;
-    if ((hasX = code_seen('X'))) x = code_value();
-    if ((hasY = code_seen('Y'))) y = code_value();
-    if ((hasZ = code_seen('Z'))) z = code_value();
+    int8_t px, py;
+    float z = 0;
+    bool hasX, hasY, hasZ, hasI, hasJ;
+    if ((hasX = code_seen('X'))) px = mbl.probe_index_x(code_value());
+    if ((hasY = code_seen('Y'))) py = mbl.probe_index_y(code_value());
     if ((hasI = code_seen('I'))) px = code_value();
     if ((hasJ = code_seen('J'))) py = code_value();
+    if ((hasZ = code_seen('Z'))) z = code_value();
 
     if (hasX && hasY && hasZ) {
 
-      px = mbl.probe_index_x(x);
-      py = mbl.probe_index_y(y);
-
       if (px >= 0 && py >= 0)
         mbl.set_z(px, py, z);
       else {
     }
     else {
       SERIAL_ERROR_START;
-      SERIAL_ERRORLNPGM(MSG_ERR_M421_REQUIRES_XYZ);
+      SERIAL_ERRORLNPGM(MSG_ERR_M421_PARAMETERS);
     }
   }
 

Marlin/language.h

 #define MSG_Z2_MAX                          "z2_max: "
 #define MSG_Z_PROBE                         "z_probe: "
 #define MSG_ERR_MATERIAL_INDEX              "M145 S<index> out of range (0-1)"
-#define MSG_ERR_M421_REQUIRES_XYZ           "M421 requires XYZ or IJZ parameters"
+#define MSG_ERR_M421_PARAMETERS             "M421 requires XYZ or IJZ parameters"
 #define MSG_ERR_MESH_XY                     "Mesh XY or IJ cannot be resolved"
 #define MSG_ERR_M428_TOO_FAR                "Too far from reference point"
 #define MSG_ERR_M303_DISABLED               "PIDTEMP disabled"