Removes a bit of unused code. This effectively reverts upstream's
25af7a5dbc. It's new with OpenSSL 1.0.2 so
nothing can be using it yet. We can restore it with tests if we end up wanting
it later.

(Also I think it might be misnamed. The KDF seems to be defined in X9.63, not
X9.62.)

Change-Id: I482daf681e0cf5c3bbdc72c57793f91448deaee8
Reviewed-on: https://boringssl-review.googlesource.com/2846
Reviewed-by: Adam Langley <agl@google.com>