Only perform the extra purge for user-requested unloads

Swaps are triggered with a hot printer, and thus don't require to
perform an extra purge. This follows my own advice from PR prusa3d#3121.

Firmware/ultralcd.cpp

@@ -6288,7 +6288,7 @@ void unload_filament(UnloadType unload)
 	lcd_setstatuspgm(_T(MSG_UNLOADING_FILAMENT));
 
     raise_z_above(unload == UnloadType::Swap? MIN_Z_FOR_SWAP: MIN_Z_FOR_UNLOAD);
-    if (unload != UnloadType::Runout)
+    if (unload == UnloadType::User)
     {
         // extrude slowly
         current_position[E_AXIS] += FILAMENTCHANGE_UNLOADFEED;