Kill mime_sniffer histograms which are unowned and unused.
Kill mime_sniffer histograms which are unowned and unused.

These are in the top 10 histograms that don't have owners
and are unused (not viewed from dashboards).

Similar to the nosniff ones I removed here:
https://codereview.chromium.org/1718613002

BUG=387734

Review-Url: https://codereview.chromium.org/1959153002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#392434}

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