Remove redundant s->server assignments in handshake.

It should be set correctly prior to entering the handshake. Don't mask bugs by
assigning it.

Change-Id: Ib9bca8fad68916b3b242aad8819e3760e59e777a
Reviewed-on: https://boringssl-review.googlesource.com/2443
Reviewed-by: Adam Langley <agl@google.com>