Move ECC extensions out of SSL_SESSION.

There's no need to store them on the session. They're temporary handshake
state and weren't serialized in d2i_SSL_SESSION anyway.

Change-Id: I830d378ab49aaa4fc6c4c7a6a8c035e2263fb763
Reviewed-on: https://boringssl-review.googlesource.com/1990
Reviewed-by: Adam Langley <agl@google.com>