Deprecate unused ScalarUDF::display_name

[findepi]

It looks the last usage was removed in

• 7c41323 / Rename Expr::display_name to Expr::schema_name, make UNNEST naming conform to convention #11797

i.e. in 42.0.0

• part of Simplify the name function in datafusion #11782

[alamb]

Makes sense to me

Looks like it was added by @yyy1000 in

• #10417

cc @jayzhan211

There are a bunch of implementations in the DataFusion function library, as you can see here: https://github.com/search?q=repo%3Aapache%2Fdatafusion%20display_name&type=code

However I didn't see any uses either

[findepi]

Maybe the "usage" is here, in a TODO comment

datafusion/datafusion/expr/src/expr.rs

Line 3252 in 541a67d

// write!(f, "{}", func.display_name(args).unwrap())

We need to decide what to do.
Keep display_name() code that's dead/redundant for a year now.
Or proceed with removal. Then the above comment should be gone.

As #11782, #10364 (comment) note, having many "name" functions is hard to follow, so I'm inclined to proceed with removal.

[alamb]

Or proceed with removal. Then the above comment should be gone.

I think we should remove it

maybe since the deprecation notice doesn't actually generate any warnings, we should just remove the function (with a breaking API change) immediately 🤔

[findepi]

right, for folks implementing ScalarUDFImpl it's a breakage sooner or later
for folks calling ScalarUDFImpl::display_name or ScalarUDF::display_name, it's a graceful deprecation with a warning, so still a bonus

[findepi]

Or proceed with removal. Then the above comment should be gone.

I think we should remove it

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