The only accessor for this field is the group/curve ID. Switch to only
storing that so no cipher checks are needed to interpret it. Instead,
ignore older values at parse time.

Change-Id: Id0946d4ac9e7482c69e64cc368a9d0cddf328bd3
Reviewed-on: https://boringssl-review.googlesource.com/12693
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>