--log-regexp was removed in
https://codereview.chromium.org/2422593003. This commit removes
references to that flag in src/logging/log.h.

Change-Id: Idc965f06dd1f85370b5391c495ae113306655b75
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2646246Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Cr-Commit-Position: refs/heads/master@{#72361}