Auto merge of #1769 - RalfJung:remove-compat, r=oli-obk

remove compatibility code for passing miri flags via cargo arguments

With https://github.com/rust-lang/miri/pull/1540, we deprecated `cargo miri test -- -Zmiri-disable-stacked-borrows` as a style of passing flags to Miri, introducing `MIRIFLAGS="-Zmiri-disable-stacked-borrows" cargo miri test` instead. This made `cargo miri` more compatible with `cargo`; both now behave the same in terms of argument parsing.

However, to avoid breaking things, I introduced some backwards compatibility hack such that the old way would still work. Six months later, I think it is time to remove that hack.

diff --cc cargo-miri/bin.rs
Simple merge