BUG/MINOR: ssl: remove dead code in ssl_sock_from_buf()

BUG/MINOR: ssl: remove dead code in ssl_sock_from_buf()

When haproxy is compiled in -O0, the SSL_get_max_early_data() symbol is
used in the generated assembly, however -O2 seems to remove this symbol
when optimizing the code.

It happens because `if conn_is_back(conn)` and `if
(objt_listener(conn->target))` are opposed conditions, which mean we
never use the branch when objt_listener(conn->target) is true.

This patch removes the dead code. Bonus: SSL_get_max_early_data() is not
implemented in rustls, and that's the only thing preventing to start
with it.

This can be backported in every stable branches.