don't pass integer by reference, inefficient. (PR#2547)

Marlin/planner.cpp

 // mm. Microseconds specify how many microseconds the move should take to perform. To aid acceleration
 // calculation the caller must also provide the physical length of the line in millimeters.
 #if defined(ENABLE_AUTO_BED_LEVELING) || defined(MESH_BED_LEVELING)
-  void plan_buffer_line(float x, float y, float z, const float &e, float feed_rate, const uint8_t &extruder)
+  void plan_buffer_line(float x, float y, float z, const float &e, float feed_rate, const uint8_t extruder)
 #else
-  void plan_buffer_line(const float &x, const float &y, const float &z, const float &e, float feed_rate, const uint8_t &extruder)
+  void plan_buffer_line(const float &x, const float &y, const float &z, const float &e, float feed_rate, const uint8_t extruder)
 #endif  // ENABLE_AUTO_BED_LEVELING
 {
   // Calculate the buffer head after we push this byte

Marlin/planner.h

    * Add a new linear movement to the buffer. x, y, z are the signed, absolute target position in
    * millimeters. Feed rate specifies the (target) speed of the motion.
    */
-  void plan_buffer_line(float x, float y, float z, const float &e, float feed_rate, const uint8_t &extruder);
+  void plan_buffer_line(float x, float y, float z, const float &e, float feed_rate, const uint8_t extruder);
 
   /**
    * Set the planner positions. Used for G92 instructions.
 
 #else
 
-  void plan_buffer_line(const float &x, const float &y, const float &z, const float &e, float feed_rate, const uint8_t &extruder);
+  void plan_buffer_line(const float &x, const float &y, const float &z, const float &e, float feed_rate, const uint8_t extruder);
   void plan_set_position(const float &x, const float &y, const float &z, const float &e);
 
 #endif // ENABLE_AUTO_BED_LEVELING || MESH_BED_LEVELING