Rollup merge of rust-lang#62978 - LukasKalbertodt:remove-array-impl-bootstrap-cfg, r=Mark-Simulacrum

Remove `cfg(bootstrap)` code for array implementations

In 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.

Unfortunately, the diff is pretty ugly because much of the code was indented by one level before. The change is pretty trivial, though.

PS: I did not run the full test suite locally. There are 40°C outside and 31°C inside my room. I don't want my notebook to melt. I hope that CI is green.

r? @scottmcm

Author: Centril