CLEANUP: auth: fix useless self-include of auth-t.h

Since recent include cleanups auth-t.h ended up including itself.
diff --git a/include/haproxy/auth-t.h b/include/haproxy/auth-t.h
index fc8b6ec..35a1ff6 100644
--- a/include/haproxy/auth-t.h
+++ b/include/haproxy/auth-t.h
@@ -15,7 +15,6 @@
 #define _HAPROXY_AUTH_T_H
 
 #include <haproxy/api-t.h>
-#include <haproxy/auth-t.h>
 
 #define AU_O_INSECURE	0x00000001		/* insecure, unencrypted password */