This dates back to upstream's a2a0158959.
It seems to be a remnant of those SSL_clear calls in the handshake state
machine which... were also bizarre and since gone.

Since SSL_clear is to drop the current connection but retain the
configuration, it doesn't really make sense to forbid it while you're
mid-handshake.

This removes another consumer of s->renegotiate.

BUG=429450

Change-Id: Ifac6bf11644447fd5571262bed7421684739bc39
Reviewed-on: https://boringssl-review.googlesource.com/4823
Reviewed-by: Adam Langley <agl@google.com>