arrow

MINOR: [C++] Remove misleading comment on FileKeyUnwrapper constructor (#40808)

### Rationale for this change

I added this comment in #34181, but from the discussion in https://github.com/apache/arrow/pull/40732#discussion_r1535001401, I realised this comment was incorrect. The extra overload appears to just be a convenience as a `FileKeyMaterialStore` is already constructed in `KeyToolkit::RotateMasterKeys`, but the store isn't actually used by the `FileKeyUnwrapper` in that method, as only `FileKeyUnwrapper::GetDataEncryptionKey` is called, which bypasses the store.

`RotateMasterKeys` does however rely on the `temp_key_material_store` passed to the `FileKeyWrapper` being used, which is possibly where this confusion came from.

### What changes are included in this PR?

Removes an incorrect statement from a C++ header comment.

### Are these changes tested?

NA

### Are there any user-facing changes?

No

Authored-by: Adam Reeve <adreeve@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>