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After https://crrev.com/c/1989750, a few invalidations-related prefs are
not used anymore. This CL adds code to clean them up.

Bug: 1029481
Change-Id: I56b75b442fed2275d502ba63370e48f006bc9bbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050591
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741104}