[blink scheduler] Remove EstimateLongestJankFreeTaskDuration

The longest jank-free task duration was once used in computing the
main thread scheduling policy, but that was removed in 2019 in
crrev.com/c/1411882. This removes the related dead code, since the only
current functional use is an unnecessary debug trace event.

Bug: None
Change-Id: I3b5d12fbbdf95d531f86e9f7e13d832a4e330e3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5490908
Commit-Queue: Scott Haseley <shaseley@chromium.org>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1293785}