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.

Signed-off-by: ozkanonur <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/download.rs

@@ -427,7 +427,6 @@ impl Config {
 
     fn download_toolchain(
         &self,
-        // FIXME(ozkanonur) use CompilerMetadata instead of `version: &str`
         version: &str,
         sysroot: &str,
         stamp_key: &str,