lib.systems: remove (accidental?) rust/rustc alias

I imagine this was supposed to be rustc = args.rustc, like the other
two lines.  This meant that we accepted both rust and rustc
attributes, with the same effect.  I doubt anybody was using the
undocumented, probably-accidental "rust" spelling, but we should
remove it before somebody starts.

In fact, we don't need to set rustc here at all, because no value
platforms.select could return will ever include a rustc key (unlike
the other two), so then rustc will be filled in later, when args is
merged into final.