Remove non-immediate core page load metrics

This change removes the PageLoad.Timing2.* metrics, as they have
been replaced with
PageLoad.(Paint|Document|Parse)Timing.* equivalents.

Additionally, histograms that use dom loading are removed, as it
has been pointed out that dom loading is not well spec'd and has
been marked as deprecated
(w3c/navigation-timing#13).

This change does not obsolete these histograms in histograms.xml,
as some experiments continue to depend on them and removing
them from histograms.xml makes them inaccessible in the Finch
dashboard. We'll remove these histograms from histograms.xml in
a follow up change in a few releases, when everyone has migrated
to the new metrics.

A few PageLoad.Timing2.* metrics are not removed in this change.
Those metrics will eventually need to be migrated to new names,
but for the time being they do not have new equivalents, so we
have not migrated them yet.

BUG=611740

Review-Url: https://codereview.chromium.org/2156093002
Cr-Commit-Position: refs/heads/master@{#406530}