Remove unnecessary calls to `.index` before accessing `field_info`

[cphyc]

Following #5250, accessing field_info automatically triggers the creation of the index if it isn't already created. We still have a few places where we still manually trigger it before accessing field_info, see e.g.

yt/yt/visualization/volume_rendering/tests/test_composite.py

Line 49 in 82ac140

ds.index

While not incorrect, this is not necessary anymore and should be removed.