src: remove TODO about uv errno removal

This commit removes a TODO regarding the removal of uv errno. errno
is currently used and cannot be removed so removing the comment to
avoid any confusion.

PR-URL: https://github.com/nodejs/node/pull/12536
Ref: https://github.com/nodejs/node/issues/4641


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>