Erase unused warning string for untrusted extension install
Erase unused warning string for untrusted extension install

Part of the string eraser effort.

BUG=None
R=rdevlin.cronin@chromium.org

Review URL: https://codereview.chromium.org/1345373005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#350022}

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