Unnecessary check in sparse LCP computation.

There is an unnecessary computation of either `t` of in _sais.c:701_ and _sais.c:731_

The latter looks like and `q` never exceeds `t`.

``` c
t = PLCP[j];          // accesses DELTA-value
q = RA[j+1]-RA[j];    // length difference
PLCP[j++] = p;
p -= (t > q) ? t : q; // decrease p by larger of t and q
```
