cleanup/ Remove deprecated extension notification from policy_browsertest.cc
cleanup/ Remove deprecated extension notification from policy_browsertest.cc

BUG=411568
TEST=browser_tests --gtest_filter=PolicyTest.*

Review URL: https://codereview.chromium.org/1390903006

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353502}

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