Remove extraneous 'extern' hints (#21516)

Marlin/src/module/servo.h

@@ -112,4 +112,4 @@
 #define MOVE_SERVO(I, P) servo[I].move(P)
 
 extern HAL_SERVO_LIB servo[NUM_SERVOS];
-extern void servo_init();
+void servo_init();

Marlin/src/module/tool_change.h

@@ -42,7 +42,7 @@
   extern toolchange_settings_t toolchange_settings;
 
   #if ENABLED(TOOLCHANGE_FILAMENT_SWAP)
-    extern void tool_change_prime();
+    void tool_change_prime();
   #endif
 
   #if ENABLED(TOOLCHANGE_FS_PRIME_FIRST_USED)