relicense shootout-fannkuch-redux.rs

Part of #14248

Main contributors are @pcwalton, @alexcrichton and me.  Only
@dguenther appear in git blame as a minor contribution, but it is
only adding the rust license, so removed by this relicensing.
auto merge of #14756 : TeXitoi/rust/relicense-shootout-fannkuch-redux, r=brson

Part of #14248

Main contributors are @pcwalton, @alexcrichton and me.  Only
@dguenther appear in git blame as a minor contribution, but it is
only adding the rust license, so removed by this relicensing.

@brson OK?