Shell has no way to remove/unset an env var

[RalfJung]

I went over the API list three times as I was sure I must just be missing this function, but I can't find it -- it seems like there is no way to remove an env var via Shell? That seems like an odd gap in functionality -- is there any particular reason for this or would a PR adding the missing method be accepted?

[RalfJung]

I'm pretty sure that implementation is subtly wrong (it un-does a previous Shell::set_var but does remove the env var if it is set in the OS env var list)... so completely on-brand for an AI bot :/

[RalfJung]

@matklad did you ask for this bot to post here? If not IMO we should report it.

[lnicola]

screw those closing braces

Latta AI, probably.