CLEANUP: applet: remove the now unused appctx->private field

Nobody uses it anymore. This shrinks the appctx size by 8 bytes.
diff --git a/include/types/applet.h b/include/types/applet.h
index e7703b9..29f6f5c 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -58,7 +58,6 @@
 	int (*io_handler)(struct appctx *appctx);  /* used within the cli_io_handler when st0 = CLI_ST_CALLBACK */
 	void (*io_release)(struct appctx *appctx);  /* used within the cli_io_handler when st0 = CLI_ST_CALLBACK,
 	                                               if the command is terminated or the session released */
-	void *private;
 	struct buffer_wait buffer_wait; /* position in the list of objects waiting for a buffer */
 
 	union {