Remove some ? from generated code

[Mingun]

This PR removes one FIXME in the code and probably slightly improves compilation time in some cases

[dtolnay]

This codepath for the zero variant case is here in order for the generated code to compile with rust older than 1.82. After this PR it would not compile:

error[E0004]: non-exhaustive patterns: `Ok(_)` not covered
 --> src/main.rs:1:10
  |
1 | #[derive(serde::Deserialize)] pub enum Void {}
  |          ^^^^^^^^^^^^^^^^^^ pattern `Ok(_)` not covered
  |
note: `Result<(__Field, <__A as EnumAccess<'_>>::Variant), <__A as EnumAccess<'_>>::Error>` defined here
 --> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/result.rs:527:1
 ::: /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/result.rs:531:5
  |
  = note: not covered
  = note: the matched value is of type `Result<(__Field, <__A as EnumAccess<'_>>::Variant), <__A as EnumAccess<'_>>::Error>`
  = note: this error originates in the derive macro `serde::Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

Serde_derive presently aims to support rust 1.61. We can probably raise that but I am not sure if 1.82 is too recent.

[Mingun]

Oh, I missed that. I removed that commit. However, it is strange why CI does not check the actual build on the minimum version (or even run tests). It only takes half a minute, is it really worth saving that time at the risk of missing breaking changes like these?