Remove DHECiphers feature flag.

This was an emergency off switch should the DHE removal not stick. That
went through fine, so remove the feature glue. The remaining glue is
still needed and will be deleted in a few cycles when the admin policy
expires.

BUG=619194

Review-Url: https://codereview.chromium.org/2458123003
Cr-Commit-Position: refs/heads/master@{#428485}
2 files changed