arrow

MINOR: [R] Trim news for 18.0 release (#44562)

### Rationale for this change

The referenced change was included in the 17.0.0.1 CRAN submission
(https://github.com/apache/arrow/issues/43317#issuecomment-2289034257)
so we don't need to announce it here. As far as CRAN releases go, the
performance regression was never released.

### What changes are included in this PR?

rm

### Are these changes tested?

🙅 

### Are there any user-facing changes?

Words