Remove P-521 from the default supported curves list.

Per review comment on https://boringssl-review.googlesource.com/#/c/2843/.

Change-Id: I84c9320ff908c9f8912e83c6ece89d9b06c32bbf
Reviewed-on: https://boringssl-review.googlesource.com/2860
Reviewed-by: Adam Langley <agl@google.com>