std::fs::path: remove unused mutability for Windows

std/fs/path/path_windows.jule

@@ -157,7 +157,7 @@ fn abs(mut path: str): (str, ok: bool) {
 	ret Clean(path), true
 }
 
-fn join(mut elem: ...str): str {
+fn join(elem: ...str): str {
 	mut s := StrBuilder.New(1 << 4)
 	mut lastChar := byte(0)
 	for (_, mut e) in elem {