Don't make SSL_MODE_*HELLO_TIME configurable.

Never send the time as a client. Always send it as a server.

Change-Id: I20c55078cfe199d53dc002f6ee5dd57060b086d5
Reviewed-on: https://boringssl-review.googlesource.com/4829
Reviewed-by: Adam Langley <agl@google.com>