PeerKey: Remove dead code related to STSL negotiation state

PeerKey: Remove dead code related to STSL negotiation state

The struct wpa_stsl_negotiation seemed to have been for some kind of
tracking of state of PeerKey negotiations within hostapd. However,
nothing is actually adding any entries to wpa_auth->stsl_negotiations or
using this state. Since PeerKey does not look like something that would
be deployed in practice, there is no justification to spend time on
making this any more complete. Remove the dead code now instead of
trying to figure out what it might be used for.

Signed-off-by: Jouni Malinen <j@w1.fi>