Remove ExpectedTaskQueueingDurationWithoutMainFrame

We are no longer using this histogram, so it can be removed.

Bug: 938171
Change-Id: I15ee0f13a82ee1f49c13e77c7ebc9aad186e9b6a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1531607
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642582}