Don't clear SSL context when TLS connection is closed.

Previous behaviour would clear the external SSL_CTX provided by
MOSQ_OPT_SSL_CTX. This required the user to reset the SSL_CTX every
disconnect, and trust that they were not leaking references.

Recreating the SSL context for every connection is not necessary, and the
SSL context is freed in mosquitto_destroy, which is sufficient.

Signed-off-by: Matthias Stone <matthias@bellstone.ca>