Remove unnecessary ERR_CHANNEL_ID_IMPORT_FAILED codepath.

This dates to https://codereview.chromium.org/338093012/, when the store
returned a string rather than an ECPrivateKey. As of
https://codereview.chromium.org/1076063002, the store returns an object so this
codepath isn't possible. Remove it.

BUG=none

Review URL: https://codereview.chromium.org/1615943003

Cr-Commit-Position: refs/heads/master@{#370984}
3 files changed