Minor: Remove now dead SUPPORTED_STRUCT_TYPES

[alamb]

Draft until #8463 is merged

Which issue does this PR close?

Follow on to #8463

Rationale for this change

#8463 removed the last use of SUPPORTED_STRUCT_TYPES so it is no longer needed in the code

What changes are included in this PR?

remove unused code

Are these changes tested?

Covered by existing tests

Are there any user-facing changes?

No

[korowa]

LGTM in DataFusion context, thank you @alamb

Just a question -- it doesn't look like a big deal for this one, but is it fine to remove public constant provided by crate without deprecating it first?

[alamb]

LGTM in DataFusion context, thank you @alamb

Just a question -- it doesn't look like a big deal for this one, but is it fine to remove public constant provided by crate without deprecating it first?

That is a good question @korowa -- I think it is a judgement call based on if we think removing the pub thing is likely to be encountered during upgrade and if the API change will be hard to adjust to for users.

In this case I don't think it will have a big impact, though I fully admit I have no data one way or the other

[metesynnada]

I think we can merge this, since it is quite minor and will not introduce a big change.