Remove support for sending V2ClientHello.

Accepting them as a server is still necessary, but this code is unreachable.
Without SSLv2 support, none of the cipher suites are SSLv2, so
ssl23_no_ssl2_ciphers always returns true and we send a V3ClientHello.

Change-Id: I09030f2c6e375660453c74e4f094d95e9908c3e1
Reviewed-on: https://boringssl-review.googlesource.com/1258
Reviewed-by: Adam Langley <agl@google.com>