Stop deriving peer finished twice for TLSv1.2.

Stop deriving peer finished twice for TLSv1.2.

We already derive the peer finished in ssl3_do_change_cipher_spec(), which
DTLS relies on. In the case of TLS we've been doing it twice - once in
ssl3_get_message() and once in ssl3_do_change_cipher_spec().

ok tb@