Remove outdated security note from A11yEvent

The note that passwords are hidden from a11y services
has led to researchers demonstrating the many ways that
they can be scraped. Remove the security note so we aren't
making a promise that we can't keep.

Bug: 38004070
Test: Comment change only, so relying on TH
Change-Id: I257f282aba7542af7543b96eb09b4f898b1a3f06