[tests] Remove non-working duplicated code

It is almost an exact copy of testrigopen 1 line different and
without includes so it's not compiling.

tests/callback.c

@@ -1,42 +0,0 @@
-int callback(const struct rig_caps *caps, rig_ptr_t rigp)
-{
-    RIG *rig = (RIG *) rigp;
-
-    rig = rig_init(caps->rig_model);
-
-    if (!rig)
-    {
-        fprintf(stderr, "Unknown rig num: %u\n", caps->rig_model);
-        fprintf(stderr, "Please check riglist.h\n");
-        exit(1); /* whoops! something went wrong (mem alloc?) */
-    }
-
-    const char *port = "/dev/pts/3";
-    strcpy(RIGPORT(rig)->pathname, port);
-
-    printf("%20s:", caps->model_name);
-    fflush(stdout);
-    struct timeval start, end;
-    gettimeofday(&start, NULL);
-    rig_open(rig);
-    gettimeofday(&end, NULL);
-    double dstart = start.tv_sec + start.tv_usec / 1e6;
-    double dend = end.tv_sec + end.tv_usec / (double)1e6;
-    printf(" %.1f\n",  dend - dstart);
-
-    rig_close(rig); /* close port */
-    rig_cleanup(rig); /* if you care about memory */
-    return 1;
-}
-
-int main(int argc, char *argv[])
-{
-    RIG rig;
-    printf("testing rig timeouts when rig powered off\n");
-
-    /* Turn off backend debugging output */
-    rig_set_debug_level(RIG_DEBUG_NONE);
-    rig_load_all_backends();
-    rig_list_foreach(callback, &rig);
-    return 0;
-}