Delete an old comment.

Change-Id: Iafd06023ceaf5023a99a0d54be846d6ae8ab1035
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62810
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>