Remove `-Zmiri-retag-fields`?

[RalfJung]

-Zmiri-retag-fields has been the default for 2 years now. I think -Zmiri-retag-fields=scalar should definitely be removed; its behavior depends on internal choices of the rustc layout algorithm that really shouldn't affect whether code has UB.

What remains then is -Zmiri-retag-fields=none -- do we need the ability to disable field retagging? Honestly I am not convinced we do. The flag is clearly unsound, under all aliasing models we are realistically going to pick. If someone doesn't want to check for aliasing violations they can always use -Zmiri-disable-stacked-borrows.

@rust-lang/miri what do you think?

[oli-obk]

yea, I think just having the big hammer for disabling stacked borrows (or tree borrows) is sufficient

[saethlin]

Agreed. Remove it.