transport: distinguish between unknown and unsupported transports

Even when built without a SSH support, we know about this transport. It
is implemented, but the current code makes us return an error message
saying it's not.

This is a leftover from the initial implementation of the transports
when there were in fact transports we knew about but were not
implemented.

Instead, let the SSH transport itself say it cannot run, the same as we
do for HTTPS.

src/transport.c

@@ -20,25 +20,16 @@ typedef struct transport_definition {
 
 static git_smart_subtransport_definition http_subtransport_definition = { git_smart_subtransport_http, 1 };
 static git_smart_subtransport_definition git_subtransport_definition = { git_smart_subtransport_git, 0 };
-#ifdef GIT_SSH
 static git_smart_subtransport_definition ssh_subtransport_definition = { git_smart_subtransport_ssh, 0 };
-#endif
 
 static transport_definition local_transport_definition = { "file://", git_transport_local, NULL };
-#ifdef GIT_SSH
-static transport_definition ssh_transport_definition = { "ssh://", git_transport_smart, &ssh_subtransport_definition };
-#else
-static transport_definition dummy_transport_definition = { NULL, git_transport_dummy, NULL };
-#endif
 
 static transport_definition transports[] = {
 	{ "git://",   git_transport_smart, &git_subtransport_definition },
 	{ "http://",  git_transport_smart, &http_subtransport_definition },
 	{ "https://", git_transport_smart, &http_subtransport_definition },
 	{ "file://",  git_transport_local, NULL },
-#ifdef GIT_SSH
 	{ "ssh://",   git_transport_smart, &ssh_subtransport_definition },
-#endif
 	{ NULL, 0, 0 }
 };
 
@@ -95,11 +86,6 @@ static int transport_find_fn(
 	if (!definition && strrchr(url, ':')) {
 		// re-search transports again with ssh:// as url so that we can find a third party ssh transport
 		definition = transport_find_by_url("ssh://");
-#ifndef GIT_SSH
-		if (!definition) {
-			definition = &dummy_transport_definition;
-		}
-#endif
 	}
 
 #ifndef GIT_WIN32
@@ -121,15 +107,6 @@ static int transport_find_fn(
  * Public API *
  **************/
 
-int git_transport_dummy(git_transport **transport, git_remote *owner, void *param)
-{
-	GIT_UNUSED(transport);
-	GIT_UNUSED(owner);
-	GIT_UNUSED(param);
-	giterr_set(GITERR_NET, "This transport isn't implemented. Sorry");
-	return -1;
-}
-
 int git_transport_new(git_transport **out, git_remote *owner, const char *url)
 {
 	git_transport_cb fn;
@@ -246,7 +223,7 @@ int git_remote_supported_url(const char* url)
 	if (transport_find_fn(&fn, url, &param) < 0)
 		return 0;
 
-	return fn != &git_transport_dummy;
+	return 1;
 }
 
 int git_transport_init(git_transport *opts, unsigned int version)