Remove disable-signin-frame-client-cert-user-selection switch

The --disable-signin-frame-client-cert-user-selection switch was only
necessary during rollout of the client certificates on the Chrome OS
sign-in screen feature.

Remove it.

Bug: 1011602
Test: browser_tests still pass
Change-Id: I79f7f8babe128d55bfc76c27272a1c020f230dfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2014446
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734892}
4 files changed