The SDK now includes a Rust version with the aarch64 cross-compilation
libraries and the toolchain job doesn't build it anymore. Yet it was
still recompiled because the path had changed.
Remove the adjustment of the download URL and any automatic building
of Rust. Just issue a warning so that any problem can be spotted easily.
This change does not affect the SDK bootstrapping (full or just stage4)
but affects ./build_packages and the toolchains job. For the toolchains
job the crossdev setup is missing anyway and rebuilding wouldn't help
but only downloading, yet since in stage4 there are no binary package
URLs at all, it's best to remove this step and if it is needed later,
the warning will help.
cfdb7b0ab7