Remove cfg(bootstrap) code for array implementations

In PR rust-lang#62435 ("Use const generics for array impls [part 1]") the old
macro-based implementations were not removed but still used with
`cfg(bootstrap)` since the bootstrap compiler had some problems with
const generics at the time. This does not seem to be the case anymore,
so there is no reason to keep the old code.

Author: LukasKalbertodt