Scrub the WipeTower2 toolchange entry with the BBL flat-ironing spiral

The entry scrub now matches the BBL tower's toolchange_wipe_new sequence:
after the ironing drag the retracted nozzle runs a dry expanding-square
spiral centred on the wall-gap entry point before resuming the purge row.
The spiral runs whenever the gap wall is on (disable per filament via
filament_tower_ironing_area = 0); WipeTower2 no longer reads
prime_tower_flat_ironing.

src/libslic3r/GCode/WipeTower2.hpp

@@ -221,7 +221,6 @@ private:
 	float  m_perimeter_speed    = 0.f;
     float  m_first_layer_speed  = 0.f;
     size_t m_first_layer_idx    = size_t(-1);
-    bool   m_flat_ironing       = false;
     bool   m_enable_tower_interface_features = false;
     bool   m_enable_tower_interface_cooldown_during_tower = false;
     bool   m_prev_layer_had_interface = false;