Remove dependency on farming (#219)

Avoids circular dependencies of 3rd party mods with unified_inventory -> farming -> stairs.

mod.conf

@@ -1,6 +1,6 @@
 name = unified_inventory
 
-optional_depends = default, creative, sfinv, datastorage, farming
+optional_depends = default, creative, sfinv, datastorage
 description = """
 Unified Inventory replaces the default survival and creative inventory.
 It adds a nicer interface and a number of features, such as a crafting guide.