remove some GCC warnings about unused variables

Fix some GCC warnings about unused variables in `src/cs/cs_qr.c` and `src/scg.c`.  This addresses some part of issue #297.  The specific GCC warnings that this patch fixes are:

```
cs/cs_qr.c:29:21: warning: variable 'm' set but not used [-Wunused-but-set-variable]
     CS_INT i, k, p, m, n, vnz, p1, top, m2, len, col, rnz, *s, *leftmost, *Ap, *Ai,
                     ^

scg.c:901:7: warning: variable 'no_of_groups' set but not used [-Wunused-but-set-variable]
   int no_of_groups;
       ^
```

I have another bunch of warning fixes for files under `src/CHOLMOD` if you're interested.