[flex] Remove use-counter for kFlexboxAlignSingleLineDifference

This was added for:
https://github.com/w3c/csswg-drafts/issues/3052

The CSSWG hasn't ended up looking at the resulting data - but likely
still not web compatible as >1% of page loads.

Change-Id: Ib227bce41aab610d3004ff9ebfb273c69e0faf9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5867711
Commit-Queue: David Grogan <dgrogan@chromium.org>
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1356647}