apps/system/composite: Restore USB tracing; remove unused field in a structure.

system/composite/composite.h

@@ -186,7 +186,6 @@ struct composite_state_s
    */
 
   FAR void *cmphandle;        /* Composite device handle */
-  FAR void *mschandle;        /* Mass storage device handle */
 
   /* Serial file descriptors */
 

system/composite/composite_main.c

@@ -549,6 +549,10 @@ int conn_main(int argc, char *argv[])
       return EXIT_FAILURE;
     }
 
+  /* Initialize USB trace output IDs */
+
+  usbtrace_enable(TRACE_BITSET);
+
 #ifdef CONFIG_SYSTEM_COMPOSITE_DEBUGMM
 #  ifdef CONFIG_CAN_PASS_STRUCTS
   g_composite.mmstart    = mallinfo();