Fix classic_zombies map_extras

Classic zombies was setting the weight of various map_extras to zero.
This makes sense, but it was also setting the chance of map extras on
certain extras types for every region.  This is problematic because some
regions' extras types lacked any map extras, so setting the chance
non-zero led to errors.

Change the classic zombies mod to just set the extras weights and not
set the chance, so that the regions with no extras would remain at the
default chance of zero and no errors would arise.

As a side-effect, this dramatically increases the chance of field map
extras for the classic zombies mod which had become out of sync with the
base game, which is probably an improvement.

data/mods/classic_zombies/exclusions.json

@@ -19,14 +19,14 @@
     "type": "region_overlay",
     "regions": [ "all" ],
     "map_extras": {
-      "forest": { "chance": 20, "extras": { "mx_portal": 0, "mx_portal_in": 0, "mx_spider": 0 } },
-      "forest_thick": { "chance": 20, "extras": { "mx_portal": 0, "mx_portal_in": 0, "mx_shia": 0, "mx_spider": 0, "mx_jabberwock": 0 } },
-      "forest_water": { "chance": 20, "extras": { "mx_portal": 0, "mx_portal_in": 0, "mx_spider": 0 } },
-      "field": { "chance": 90, "extras": { "mx_portal": 0, "mx_portal_in": 0 } },
-      "road": { "chance": 75, "extras": { "mx_portal": 0, "mx_portal_in": 0 } },
-      "build": { "chance": 90, "extras": { "mx_house_spider": 0, "mx_house_wasp": 0, "mx_portal": 0, "mx_portal_in": 0 } },
+      "forest": { "extras": { "mx_portal": 0, "mx_portal_in": 0, "mx_spider": 0 } },
+      "forest_thick": { "extras": { "mx_portal": 0, "mx_portal_in": 0, "mx_shia": 0, "mx_spider": 0, "mx_jabberwock": 0 } },
+      "forest_water": { "extras": { "mx_portal": 0, "mx_portal_in": 0, "mx_spider": 0 } },
+      "field": { "extras": { "mx_portal": 0, "mx_portal_in": 0 } },
+      "road": { "extras": { "mx_portal": 0, "mx_portal_in": 0 } },
+      "build": { "extras": { "mx_house_spider": 0, "mx_house_wasp": 0, "mx_portal": 0, "mx_portal_in": 0 } },
       "marloss": { "chance": 0, "extras": { "mx_marloss_pilgrimage": 0 } },
-      "subway": { "chance": 75, "extras": { "mx_portal": 0, "mx_portal_in": 0 } }
+      "subway": { "extras": { "mx_portal": 0, "mx_portal_in": 0 } }
     }
   }
 ]