Remove ec hybrid point format

According to rfc5480 and rfc4492 the hybrid format is not allowed
neither in certificates or the tls protocol.

Change-Id: I1d3fb5bef765bc7b58d29bdd60e15247fac4dc7a
Reviewed-on: https://boringssl-review.googlesource.com/2510
Reviewed-by: Adam Langley <agl@google.com>