CLEANUP: applet: rename struct si_applet to applet

Since this one does not depend on stream_interface anymore, remove the
"si_" prefix.
diff --git a/include/proto/applet.h b/include/proto/applet.h
index 61bca02..fd458d4 100644
--- a/include/proto/applet.h
+++ b/include/proto/applet.h
@@ -42,7 +42,7 @@
  * pool_free2(connection) or appctx_free(), since it's allocated from the
  * connection pool. <applet> is assigned as the applet, but it can be NULL.
  */
-static inline struct appctx *appctx_new(struct si_applet *applet)
+static inline struct appctx *appctx_new(struct applet *applet)
 {
 	struct appctx *appctx;