Simplify DRY RUN (PR#2358)

Elsewhere DRYRUN turns off the heating elements
and ignores constraints on them.

Here, whenever motion is entered into the planner,
if DRY RUN is set, we instantly act as if the E_AXIS
is in the desired final position.

Marlin/planner.cpp

@@ -502,12 +502,17 @@ float junction_deviation = 0.1;
 
   float dx = target[X_AXIS] - position[X_AXIS],
         dy = target[Y_AXIS] - position[Y_AXIS],
-        dz = target[Z_AXIS] - position[Z_AXIS],
+        dz = target[Z_AXIS] - position[Z_AXIS];
-        de = target[E_AXIS] - position[E_AXIS];
+
+  // DRYRUN ignores all temperature constraints and assures that the extruder is instantly satisfied
+  if (marlin_debug_flags & DEBUG_DRYRUN)
+    position[E_AXIS] = target[E_AXIS];
+
+  float de = target[E_AXIS] - position[E_AXIS];
 
   #ifdef PREVENT_DANGEROUS_EXTRUDE
     if (de) {
-      if (degHotend(extruder) < extrude_min_temp && !(marlin_debug_flags & DEBUG_DRYRUN)) {
+      if (degHotend(extruder) < extrude_min_temp) {
         position[E_AXIS] = target[E_AXIS]; // Behave as if the move really took place, but ignore E part
         de = 0; // no difference
         SERIAL_ECHO_START;