Remove kPermuteTLSExtensions feature flag

This launched a long time ago.

Change-Id: I355d638847c8a6b2380a51d0ccba5ce61ebfb286
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5482989


Auto-Submit: David Benjamin <davidben@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1291979}