Consider removing "Not in HLO" ops from VHLO

[burmako]

Given that the "Not in HLO" ops are on their way out of StableHLO, I'm wondering if we should remove them from VHLO and fail when encountering them in stablehlo-legalize-to-vhlo requiring that these ops are lowered to core ops in the opset before being serialized.

[burmako]

Upon further thinking, this seems premature at this point. For better or for worse, these ops are part of StableHLO at the moment, so removing them from VHLO without any replacement would create an unnecessary friction point.

Once the ops are moved out of StableHLO (#3), we'll be able to deprecate and eventually remove them from VHLO using the regular process. #3 is part of the Frontend Contract workstream, which means that we're planning to get this done prior to releasing StableHLO v1.0.