This field was added in order to help us track down some of the memory
leaks which we have already found, and the field didn't turn out to be
that useful. When it was introduced, the implications on the increase in
allocations were unknown, and now I don't think the field is worth it.

```
name                                           old time/op    new time/op    delta
FlowSetup/vectorize=true/distribute=true-24       168µs ± 5%     164µs ± 4%  -2.33%  (p=0.007 n=19+20)
FlowSetup/vectorize=true/distribute=false-24      167µs ± 6%     164µs ± 6%    ~     (p=0.060 n=20+20)
FlowSetup/vectorize=false/distribute=true-24      163µs ± 4%     161µs ± 7%    ~     (p=0.057 n=19+20)
FlowSetup/vectorize=false/distribute=false-24     161µs ± 6%     159µs ± 5%    ~     (p=0.309 n=19+20)

name                                           old alloc/op   new alloc/op   delta
FlowSetup/vectorize=true/distribute=true-24      19.6kB ± 8%    19.0kB ± 8%  -2.62%  (p=0.001 n=19+18)
FlowSetup/vectorize=true/distribute=false-24     18.2kB ± 1%    17.7kB ± 1%  -2.56%  (p=0.000 n=17+16)
FlowSetup/vectorize=false/distribute=true-24     25.8kB ± 2%    25.4kB ± 0%  -1.44%  (p=0.000 n=16+16)
FlowSetup/vectorize=false/distribute=false-24    24.7kB ± 0%    24.4kB ± 1%  -1.36%  (p=0.000 n=16+16)

name                                           old allocs/op  new allocs/op  delta
FlowSetup/vectorize=true/distribute=true-24         218 ± 2%       205 ± 3%  -5.64%  (p=0.000 n=19+19)
FlowSetup/vectorize=true/distribute=false-24        208 ± 1%       197 ± 3%  -5.63%  (p=0.000 n=19+19)
FlowSetup/vectorize=false/distribute=true-24        206 ± 0%       197 ± 0%  -4.40%  (p=0.000 n=16+16)
FlowSetup/vectorize=false/distribute=false-24       197 ± 0%       188 ± 0%  -4.54%  (p=0.000 n=16+16)
```

Release justification: low-risk cleanup.

Release note: None