Remove enable_wifi_bootstrapping from doc

Feature does not exists.

BUG=451851

Review URL: https://codereview.chromium.org/1575493003

Cr-Original-Commit-Position: refs/heads/master@{#370437}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: d039a9f470fa9795ebae8516b9a88b8557ee6d17