Remove special case code for SCTP reneg handling

There was code existing which attempted to handle the case where application
data is received after a reneg handshake has started in SCTP. In normal DTLS
we just fail the connection if this occurs, so there doesn't seem any reason
to try and work around it for SCTP. In practice it didn't work properly
anyway and is probably a bad idea to start with.

Fixes #3251

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3287)