Bug 2059493: Clean up components in perfcompare frontend using the unecessary silverman_kde data

[kala-moz]

Bug 2059493

Removes unused Mann-Whitney-U statistics that the frontend no longer reads, so the backend can stop computing and sending them. Two independent cleanups:

1. Drop unused MannWhitneyResultsItem fields

Fields that existed only in the type and test fixtures — never read anywhere in the app:

kde_new, 
kde_base, 
is_fit_good, 
is_significant, 
performance_intepretation (sic), 
warning_c_delta (plus the now-dead KDEItem type).

2. Remove the Silverman KDE feature end-to-end

Drop the silverman_kde field (+ dead SilvermanKDEItem/ModeItem types), remove the Distribution / Estimated Modes rows from the expanded metrics panel, and delete the getModeInterpretation helper that fed them.
Stop sending enable_silverman_kde — remove the silvermanKDEEnabled plumbing from treeherder.ts, all six loaders, useSubtestRegressionCount, and ResultsTable.

Intentionally kept

silverman_warnings (still surfaced by StatisticsWarnings) — separate follow-up if needed.

Notes for the backend

One spelling gotcha: the field is performance_intepretation (missing an "r").

🤖 Generated with Claude's help

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