Allow odd mm bed sizes (#15258)

Marlin/src/inc/Conditionals_post.h

 #endif
 
 // Define center values for future use
+#define _X_HALF_BED ((X_BED_SIZE) / 2)
+#define _Y_HALF_BED ((Y_BED_SIZE) / 2)
 #if ENABLED(BED_CENTER_AT_0_0)
   #define X_CENTER 0
   #define Y_CENTER 0
 #else
-  #define X_CENTER ((X_BED_SIZE) / 2)
-  #define Y_CENTER ((Y_BED_SIZE) / 2)
+  #define X_CENTER _X_HALF_BED
+  #define Y_CENTER _Y_HALF_BED
 #endif
-#define Z_CENTER ((Z_MIN_POS + Z_MAX_POS) / 2)
 
 // Get the linear boundaries of the bed
-#define X_MIN_BED (X_CENTER - (X_BED_SIZE) / 2)
-#define X_MAX_BED (X_CENTER + (X_BED_SIZE) / 2)
-#define Y_MIN_BED (Y_CENTER - (Y_BED_SIZE) / 2)
-#define Y_MAX_BED (Y_CENTER + (Y_BED_SIZE) / 2)
+#define X_MIN_BED (X_CENTER - _X_HALF_BED)
+#define X_MAX_BED (X_MIN_BED + X_BED_SIZE)
+#define Y_MIN_BED (Y_CENTER - _Y_HALF_BED)
+#define Y_MAX_BED (Y_MIN_BED + Y_BED_SIZE)
 
 /**
  * Dual X Carriage