Simplify subgroup feature controls

[andyleiserson]

A couple possible simplifications that came up in working on #8203 and discussion in today's maintainers meeting:

• Naga has an enum SubgroupOperationSet that finely classifies the supported subgroup operations. It is not clear that this is necessary and non-standard configurations are not well tested. Consider removing it.
• There is logic to set Capabilities::SUBGROUP whenever Features::SUBGROUP_VERTEX is set.

  wgpu/wgpu-core/src/device/mod.rs

  Line 495 in 1791064

  features.intersects(wgt::Features::SUBGROUP | wgt::Features::SUBGROUP_VERTEX),

  Although it is likely true that the fragment/compute subgroup support governed by Capabilities::SUBGROUP is available whenever Feature::SUBGROUP_VERTEX is, it seems simpler not to do this kind of mapping of feature enables behind the scenes due to the risk of confusion. It would be better if the Capabilities and Features aligned exactly.