[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 #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

Author: kovdan01

test/AutoDiff/Sema/differentiable_attr_type_checking.swift

@@ -290,15 +290,13 @@ public struct PublicDiffAttrConformance: ProtocolRequirements {
   var x: Float
   var y: Float
 
-  // FIXME(TF-284): Fix unexpected diagnostic.
   // expected-note @+2 {{candidate is missing explicit '@differentiable(reverse)' attribute to satisfy requirement}} {{10-10=@differentiable(reverse) }}
   // expected-note @+1 {{candidate has non-matching type '(x: Float, y: Float)'}}
   public init(x: Float, y: Float) {
     self.x = x
     self.y = y
   }
 
-  // FIXME(TF-284): Fix unexpected diagnostic.
   // expected-note @+2 {{candidate is missing explicit '@differentiable(reverse)' attribute to satisfy requirement}} {{10-10=@differentiable(reverse) }}
   // expected-note @+1 {{candidate has non-matching type '(x: Float, y: Int)'}}
   public init(x: Float, y: Int) {