Remove support for miri_get_backtrace with flags == 0

[RalfJung]

The main user here is the backtrace crate and it has used the v1 protocol for a while now, so we can remove support for the old protocol.

[tiif]

I'd probably get to this soon, but in case someone wants to pick it up, there are more instructions here. #4122 (comment)

@rustbot label +C-cleanup +C-enhancement

[RalfJung]

#4172 takes care of this