Function decl. in headers don't need "extern"

Marlin/Marlin.h

 extern bool axis_homed[3]; // axis[n].is_homed
 
 // GCode support for external objects
-extern bool code_seen(char);
-extern float code_value();
-extern long code_value_long();
-extern int16_t code_value_short();
+bool code_seen(char);
+float code_value();
+long code_value_long();
+int16_t code_value_short();
 
 #if ENABLED(DELTA)
   extern float delta[3];
 // Handling multiple extruders pins
 extern uint8_t active_extruder;
 
-#if ENABLED(DIGIPOT_I2C)
-  extern void digipot_i2c_set_current(int channel, float current);
-  extern void digipot_i2c_init();
-#endif
-
 #if HAS_TEMP_HOTEND || HAS_TEMP_BED
   void print_heaterstates();
 #endif
 
-extern void calculate_volumetric_multipliers();
+void calculate_volumetric_multipliers();
 
 #endif //MARLIN_H

Marlin/Marlin_main.cpp

 }
 #endif //!SDSUPPORT
 
+#if ENABLED(DIGIPOT_I2C)
+  extern void digipot_i2c_set_current(int channel, float current);
+  extern void digipot_i2c_init();
+#endif
+
 /**
  * Inject the next "immediate" command, when possible.
  * Return true if any immediate commands remain to inject.