Prerender: Remove OnCancelPrerenderHandle()
Prerender: Remove OnCancelPrerenderHandle()

This artefact has never been used. It was added here in 2012:
https://chromiumcodereview.appspot.com/10553029

BUG=none

Review-Url: https://codereview.chromium.org/1987693002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#394138}

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