backend/local: fix "operation not supported" when unlocking

If the repo is on a mounted folder that doesn't support chmod (like
SMB), it was causing an "operation not supported" error when trying to
chmod 666 a file before deleting it.

But it isn't generally needed before deleting a file (the folder
permissions matter there, not the file permissions). So, just drop it.

internal/backend/local/local_unix.go

@@ -52,3 +52,7 @@ func setFileReadonly(f string, mode os.FileMode) error {
 
 	return err
 }
+
+func removeFile(f string) error {
+	return os.Remove(f)
+}