Drop unnecessary version checks.

These may as well be replaced with assertions. Get them out of the way of the
initialization.

Change-Id: Ie4ab8bdc018e4a1def7d3f6b3b172a77896bfc0a
Reviewed-on: https://boringssl-review.googlesource.com/2563
Reviewed-by: Adam Langley <agl@google.com>