Move common strings (#20846)

Marlin/src/gcode/geometry/M206_M428.cpp

@@ -30,8 +30,6 @@
 #include "../../libs/buzzer.h"
 #include "../../MarlinCore.h"
 
-extern const char SP_Y_STR[], SP_Z_STR[];
-
 void m206_report() {
   SERIAL_ECHOLNPAIR_P(PSTR("M206 X"), home_offset.x, SP_Y_STR, home_offset.y, SP_Z_STR, home_offset.z);
 }