dda_queue.c/.h: eliminate queue_current_movement().

Again no stepping performance improvement, but another 34 bytes
off the binary size:

  ATmega sizes               '168   '328(P)   '644(P)     '1280
  Program:  19364 bytes      136%       64%       31%       16%
     Data:   2179 bytes      213%      107%       54%       27%
   EEPROM:     32 bytes        4%        2%        2%        1%

dda.c

@@ -764,7 +764,9 @@ void dda_clock() {
   uint8_t current_id ;
   #endif
 
-  dda = queue_current_movement();
+  ATOMIC_START
+    dda = mb_tail_dda;
+  ATOMIC_END
   if (dda != last_dda) {
     move_state.debounce_count_x =
     move_state.debounce_count_z =

dda_queue.c

@@ -51,20 +51,6 @@ uint8_t queue_full() {
   return MB_NEXT(mb_head) == mb_tail;
 }
 
-/// Return the current movement, or NULL, if there's no movement going on.
-DDA *queue_current_movement() {
-  DDA* current;
-
-  ATOMIC_START
-    current = &movebuffer[mb_tail];
-
-    if ( ! current->live)
-      current = NULL;
-  ATOMIC_END
-
-  return current;
-}
-
 // -------------------------------------------------------
 // This is the one function called by the timer interrupt.
 // It calls a few other functions, though.

dda_queue.h

@@ -22,7 +22,6 @@ extern DDA *mb_tail_dda;
 
 // queue status methods
 uint8_t queue_full(void);
-DDA *queue_current_movement(void);
 
 // take one step
 void queue_step(void);