[OPS/BLOCKSPARSE] remove unnecessary mask (#1351)

This PR applies a minor patch that removes unnecessary masks in
`_dsd_kernel()`.

### Details

`offs_bn` is defined as follows and not updated after that.
```py
offs_bn = pid_m * TILE_N + tl.arange(0, TILE_N)
offs_bn = tl.max_contiguous(tl.multiple_of(offs_bn % DS0, TILE_N), TILE_N)
```

Because `offs_bn = offs_bn % DS0`, this mask is always `True`.
```py
b = tl.load(pb, mask=offs_bn[None, :] < DS0)
```
This PR removes this mask (as well as explicit `mask=True`).

python/triton/ops/blocksparse/matmul.py

@@ -181,8 +181,8 @@ def _dsd_kernel(
     inc_b = tl.load(pinc)
     inc_b = tl.multiple_of(inc_b, 8)
     for k in range(K, 0, -TILE_K):
-        a = tl.load(pa, mask=True)
-        b = tl.load(pb, mask=offs_bn[None, :] < DS0)
+        a = tl.load(pa)
+        b = tl.load(pb)
         acc += tl.dot(a, b)
         pa += inc_a
         pb += inc_b * stride_bk