Remove attestation-based enrollment success message

This makes the enrollment success screen exactly the same regardless of
how one enrolled (manuaklly or with attestation-based enrollment).

Bug: chromium:1079008
Test: manual
Change-Id: I7c6df02d151828a1ae677cd6c97a764c498dafb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2186367
Auto-Submit: Yves Arrouye <drcrash@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Yves Arrouye <drcrash@chromium.org>
Cr-Commit-Position: refs/heads/master@{#766857}