SotA: Fix and improve freeze/thaw unit implementation

Bug was that when a unit was thawed, mouse over in unit sidebar would show
incorrect movement costs (all 1's).

New implementation properly uses {UNREACHABLE} and makes use of the now
available [remove_object] tag.

data/campaigns/Secrets_of_the_Ancients/utils/sota-utils.cfg

@@ -552,73 +552,41 @@
 
 #define FREEZE_UNIT ID
     [object]
+        id=freeze_unit
+        take_only_once=no
         silent=yes
         duration=scenario  # So this object will not be in the save files forever.
         [filter]
             id={ID}
         [/filter]
         [effect]
             apply_to=movement_costs
-            replace=no  # So this amount is added to the current number.
+            replace=yes
             [movement_costs]
-                shallow_water=100
-                reef=100
-                swamp_water=100
-                flat=100
-                sand=100
-                forest=100
-                hills=100
-                mountains=100
-                village=100
-                castle=100
-                cave=100
-                frozen=100
-                unwalkable=100
-                fungus=100
+                shallow_water={UNREACHABLE}
+                reef={UNREACHABLE}
+                swamp_water={UNREACHABLE}
+                flat={UNREACHABLE}
+                sand={UNREACHABLE}
+                forest={UNREACHABLE}
+                hills={UNREACHABLE}
+                mountains={UNREACHABLE}
+                village={UNREACHABLE}
+                castle={UNREACHABLE}
+                cave={UNREACHABLE}
+                frozen={UNREACHABLE}
+                unwalkable={UNREACHABLE}
+                fungus={UNREACHABLE}
             [/movement_costs]
         [/effect]
     [/object]
 #enddef
 
 #define THAW_UNIT ID
-    [store_unit]
-        [filter]
-            id={ID}
-            # This prevents a unit from being thawed that was not frozen first:
-            movement_cost=100-120
-        [/filter]
-        variable=stored_unit
-    [/store_unit]
-    [object]
-        silent=yes
-        duration=scenario
-        [filter]
-            id={ID}
-        [/filter]
-        [effect]
-            apply_to=movement_costs
-            replace=no  # This amount is added to the current number.
-            [movement_costs]
-                shallow_water=-100
-                reef=-100
-                swamp_water=-100
-                flat=-100
-                sand=-100
-                forest=-100
-                hills=-100
-                mountains=-100
-                village=-100
-                castle=-100
-                cave=-100
-                frozen=-100
-                unwalkable=-100
-                fungus=-100
-            [/movement_costs]
-        [/effect]
-    [/object]
-    [clear_variable]
-        name=stored_unit
-    [/clear_variable]
+    [remove_object]
+        id={ID}
+        object_id=freeze_unit
+    [/remove_object]
 #enddef
 
 # Finds the nearest hex to (X,Y) within MAX_DISTANCE that matches the FILTER, and stores