Move deprecation test to SanityCheck.h

Marlin/SanityCheck.h

     #error BTENABLED has been replaced with BLUETOOTH. Please update your configuration.
   #endif
 
+  #ifdef CUSTOM_MENDEL_NAME
+    #error CUSTOM_MENDEL_NAME deprecated - use CUSTOM_MACHINE_NAME instead
+  #endif
+
 #endif //SANITYCHECK_H

Marlin/configurator/config/language.h

   #define MACHINE_NAME "3D Printer"
 #endif
 
-#ifdef CUSTOM_MENDEL_NAME
-  #error CUSTOM_MENDEL_NAME deprecated - use CUSTOM_MACHINE_NAME
-  #define CUSTOM_MACHINE_NAME CUSTOM_MENDEL_NAME
-#endif
-
 #ifdef CUSTOM_MACHINE_NAME
   #undef MACHINE_NAME
   #define MACHINE_NAME CUSTOM_MACHINE_NAME

Marlin/language.h

   #define MACHINE_NAME "3D Printer"
 #endif
 
-#ifdef CUSTOM_MENDEL_NAME
-  #error CUSTOM_MENDEL_NAME deprecated - use CUSTOM_MACHINE_NAME
-  #define CUSTOM_MACHINE_NAME CUSTOM_MENDEL_NAME
-#endif
-
 #ifdef CUSTOM_MACHINE_NAME
   #undef MACHINE_NAME
   #define MACHINE_NAME CUSTOM_MACHINE_NAME