Remove ssl2.h and ssl23.h.

The SSL23_ST_foo macros are only used in ssl_stat.c.
However, these states are never set and can be removed.

Move the two remaining SSLv2 client hello record macros to ssl3.h

Change-Id: I76055405a9050cf873b4d1cbc689e54dd3490b8a
Reviewed-on: https://boringssl-review.googlesource.com/4160
Reviewed-by: Adam Langley <agl@google.com>