Remove single-variant Guard enums.

[nnethercote]

They're not useful.

[rust-highfive]

r? @varkor

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[petrochenkov]

cc @F001
The enum was introduced in #53815 as a "first step to implement RFC 2294: if-let-guard".

Citing my comment there:

My only concern is that there's pretty long road between adding if-let guards into AST/HIR and making them actually work, so this might stay as dead code for some time.

[nnethercote]

I see. I will change this PR so it just adds a comment on the types explaining why they exist.

[bors]

☔ The latest upstream changes (presumably #58316) made this pull request unmergeable. Please resolve the merge conflicts.