Don't construct a column matrix when there's no missing value

I've spotted that there's been a `FIXME` in `column_matrix.h` since 2022 that constructs a column matrix for a DMatrix even when there's no missing value:

```c
// all columns are dense column and has no missing value
// FIXME(jiamingy): We don't need a column matrix if there's no missing value.
template
void SetIndexNoMissing(bst_idx_t base_rowid, RowBinIdxT const* row_index, const size_t n_samples,
const size_t n_features, int32_t n_threads) {
missing_.GrowTo(feature_offsets_[n_features], false);

DispatchBinType(bins_type_size_, [&](auto t) {
using ColumnBinT = decltype(t);
auto column_index = Span{reinterpret_cast(index_.data()),
static_cast(index_.size() / sizeof(ColumnBinT))};
ParallelFor(n_samples, n_threads, [&](auto rid) {
rid += base_rowid;
const size_t ibegin = rid * n_features;
const size_t iend = (rid + 1) * n_features;
for (size_t i = ibegin, j = 0; i < iend; ++i, ++j) {
const size_t idx = feature_offsets_[j];
// No need to add offset, as row index is compressed and stores the local index
column_index[idx + rid] = row_index[i];
}
});
});
}
```

In testing, this step seems to make up 30% of the time building a Quantile DMatrix, as well as contributes a decently large amount of memory consumption.

I'm very much not a CPP expert, so may be wrong in my understanding, but based on the presence of a FIXME (@trivialfis), I think a decently large performance boost can come from making this change.