Remove duplicate std::filesystem::path hash function

Recent c++ std implementations now provide this (like they should've
from the beginning...)

engine/asset/include/asset.hpp

@@ -9,15 +9,6 @@
 #include "asset_types.hpp"
 #include "string_utils.hpp"
 
-namespace std {
-    template <>
-    struct hash<prism::path> {
-        std::size_t operator()(const prism::path& k) const {
-            return (std::hash<std::string>()(k.string()));
-        }
-    };
-}
-
 template<typename T>
 std::unique_ptr<T> load_asset(const prism::path& p);