Unused parameter in copyFile function in unionFile

[nidhi-singh02]

In unionFile.go, at line no 270, copyFile function accepts a parameter named "base" https://github.com/spf13/afero/blob/5c4385aa20510dba5ca4db12c02b0c9211d82892/unionFile.go#L270C15-L270C19, which is not used.

Could be a good practice to remove it from the function definition. I could raise a PR for the same once issue is accepted.