Don't artificially limit the size of the ClientHello

We were setting a limit of SSL3_RT_MAX_PLAIN_LENGTH on the size of the
ClientHello. AFAIK there is nothing in the standards that requires this
limit.

The limit goes all the way back to when support for extensions was first
added for TLSv1.0. It got converted into a WPACKET max size in 1.1.1. Most
likely it was originally added to avoid the complexity of having to grow
the init_buf in the middle of adding extensions. With WPACKET this is
irrelevant since it will grow automatically.

This issue came up when an attempt was made to send a very large
certificate_authorities extension in the ClientHello.

We should just remove the limit.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7424)

(cherry picked from commit 7835e97b)