[AutoDiff] Delete wrong FIXMEs (TF-284) from tests (#78663)
In AutoDiff/Sema/differentiable_attr_type_checking.swift, we have a
couple of following FIXMEs:
```
// FIXME(TF-284): Fix unexpected diagnostic.
```
However, the diagnostic is expected for the case of public protocol
requirements: see description https://github.com/swiftlang/swift/pull/30629.
This PR removed the diagnostic for less-than-public-visible requirements,
and the FIXME was initially related to them.
It looks like that the FIXMEs present now are a result of copy-paste and
have no meaning, and the diagnostic is expected and should be present and
does not need to be removed.
Fixes #78609
