Revert "Add decay metrics to the site engagement service."
Revert "Add decay metrics to the site engagement service."

This reverts commit add65008c043f528d34f1a257bfcf0b4fd841c4d.

Reason for revert: this commit is a broken split-off from
https://codereview.chromium.org/1698953002. The CQ managed to land the
patch twice, with this second landing being just the histograms.xml
file and nothing else. The second landing applies cleanly and introduces
duplicate, out of order histograms.

TBR=asvitkine,calamity
BUG=588580

Review URL: https://codereview.chromium.org/1718993002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#376719}

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