MD Settings: remove listener when resetting prefs
MD Settings: remove listener when resetting prefs

This prevents the previously set API from triggering
the onPrefsChanged handler after the element is reset.

BUG=425627
R=dbeam@chromium.org

Review URL: https://codereview.chromium.org/1473433004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#361588}

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