Remove ResumeNavigationWithSpeculativeRFHProcessGone feature

Remove the unlaunched kResumeNavigationWithSpeculativeRFHProcessGone
feature flag and its associated code paths. Clean up the speculative
RFH process exit handling in RenderFrameHostManager, remove the
associated histogram and fieldtrial testing config entries, and update
affected browsertests.

OBSOLETE_HISTOGRAM[Navigation.SpeculativeRFHProcessGone.MayResume]=Removed along with the unlaunched ResumeNavigationWithSpeculativeRFHProcessGone feature.

Bug: 453878130, 569408565
Change-Id: I1b314f26d12d0a1e11d70c2d872b006cc47fcfdc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8597047
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Jiacheng Guo <gjc@google.com>
Cr-Commit-Position: refs/heads/main@{#1722512}