Remove PerformanceMonitor strings
Remove PerformanceMonitor strings

The performance monitor has been dead for awhile, but looks like these weren't
ripped up with the rest.  Kill 'em!

Review URL: https://codereview.chromium.org/1293893002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#343491}

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