Remove EXTENT.

This isn't necessary; everywhere it's used can use alternatives like
constexpr size() (which is clearer), or in the limit std::tuple_size_v.

Bug: 364987728
Change-Id: I5f3a97490765aca2fbb80963902071910b5ad397
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5994531
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/main@{#1378718}