Remove header license

[tesuji]

Related rust-lang/rust#57108.

[kinnison]

I don't understand why this is desirable considering the shell script is acquirable independently of rustup itself.

[tesuji]

No, I just saw header license in rustup-init.sh. Looks like I am wrong. In that case, I will change the issue name.

[kinnison]

Given #1733 (comment) I think @nrc is in favour of this change anyway, so I'll remove my mild objection.

[tesuji]

Closed on #1733.