arrow

GH-40858: [R] Remove dangling commas from codegen.R (#40859)

### Rationale for this change

This is a draft PR fixing https://github.com/apache/arrow/issues/40858, though I'm not sure how or why this broke (or worked correctly).

Fixes #40858

### Are these changes tested?

These have been tested locally.
* GitHub Issue: #40858

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Bryce Mecum <petridish@gmail.com>