Make the dependency on minetest_game optional

treecapitator works fine without default mod, so we do not need this flexibility-reducing hard dependency.

mod.conf

@@ -1,3 +1,2 @@
 name = treecapitator
-depends = default
-optional_depends = farming_plus, moretrees, ethereal
+optional_depends = default, farming_plus, moretrees, ethereal

trees.lua

@@ -1,6 +1,5 @@
-local mgname = minetest.get_mapgen_setting"mg_name"
-
-if mgname == "v6" then
+if minetest.get_modpath("default") then
+	if minetest.get_mapgen_setting"mg_name" == "v6" then
 		treecapitator.register_tree{
 			trees = {"default:tree"},
 			leaves = {"default:leaves"},
@@ -70,6 +69,7 @@ treecapitator.register_tree({
 		leaves = {"default:aspen_leaves"},
 		range = 4,
 	})
+end
 
 if minetest.get_modpath("farming_plus") then
 	treecapitator.register_tree({