fs/Charset: return std::string from PathFromUTF8()

Don't expose pointer that requires the caller to invoke g_free(),
because that's GLib-only.

src/fs/AllocatedPath.hxx

@@ -44,13 +44,6 @@ class AllocatedPath {
 
 	string value;
 
-	struct Donate {};
-
-	/**
-	 * Donate the allocated pointer to a new #AllocatedPath object.
-	 */
-	AllocatedPath(Donate, pointer _value);
-
 	AllocatedPath(const_pointer _value):value(_value) {}
 
 	AllocatedPath(string &&_value):value(std::move(_value)) {}