fs: refactor to remove redundant validation

The function makeCallback already calls validateCallback, so the call inside copyFile can be removed.

makeCallback is called here: https://github.com/nodejs/node/blob/568b26ac6e7c82566187a39414920c94ba20f686/lib/fs.js#L2050

and makeCallback calls validateCallback here: https://github.com/nodejs/node/blob/568b26ac6e7c82566187a39414920c94ba20f686/lib/fs.js#L182