http: move HTTP/2 cleanup code off http_disconnect()

Otherwise it would never be called for an HTTP/2 connection, which has
its own disconnect handler.

I spotted this while debugging <https://bugzilla.redhat.com/1248389>
where the http_disconnect() handler was called on an FTP session handle
causing 'dnf' to crash.  conn->data->req.protop of type (struct FTP *)
was reinterpreted as type (struct HTTP *) which resulted in SIGSEGV in
Curl_add_buffer_free() after printing the "Connection cache is full,
closing the oldest one." message.

A previously working version of libcurl started to crash after it was
recompiled with the HTTP/2 support despite the HTTP/2 protocol was not
actually used.  This commit makes it work again although I suspect the
root cause (reinterpreting session handle data of incompatible protocol)
still has to be fixed.  Otherwise the same will happen when mixing FTP
and HTTP/2 connections and exceeding the connection cache limit.

Reported-by: Tomas Tomecek
Bug: https://bugzilla.redhat.com/1248389