Remove compatibility s->version checks.

They were added to avoid accidentally enabling renego for a consumer which set
them to zero to break the handshake on renego. Now that renego is off by
default, we can get rid of them again.

Change-Id: I2cc3bf567c55c6562352446a36f2b5af37f519ba
Reviewed-on: https://boringssl-review.googlesource.com/4827
Reviewed-by: Adam Langley <agl@google.com>