Rollup merge of #111069 - ozkanonur:remove-pointless-fixme, r=albertlarsan68

remove pointless `FIXME` in `bootstrap::download`

The suggestion given by `FIXME` to use `CompilerMetadata` for `download_toolchain` in `bootstrap::download` can result in more confusion. This is because `stamp_key` is not always a date; it can also be a commit hash. Additionally, unlike in `download_beta_toolchain`, in the `download_ci_rustc` function, `version` and `commit` values are calculated separately.

Author: Dylan-DPC