Farming: Deprecate bronze, mese and diamond hoes. Tune steel uses (#2103)

Remove unnecessary "air" fallback recipe for hoes to avoid this showing
in crafting guides.

mods/farming/api.lua

@@ -83,13 +83,6 @@ farming.register_hoe = function(name, def)
 	if def.inventory_image == nil then
 		def.inventory_image = "unknown_item.png"
 	end
-	if def.recipe == nil then
-		def.recipe = {
-			{"air","air",""},
-			{"","group:stick",""},
-			{"","group:stick",""}
-		}
-	end
 	if def.max_uses == nil then
 		def.max_uses = 30
 	end
@@ -104,12 +97,12 @@ farming.register_hoe = function(name, def)
 		sound = {breaks = "default_tool_breaks"},
 	})
 	-- Register its recipe
-	if def.material == nil then
+	if def.recipe then
 		minetest.register_craft({
 			output = name:sub(2),
 			recipe = def.recipe
 		})
-	else
+	elseif def.material then
 		minetest.register_craft({
 			output = name:sub(2),
 			recipe = {