chore: remove deprecated ScalarUDF / ScalarUDFImpl methods past the API health policy

[MannXo]

Which issue does this PR close?

• Closes Remove deprecated ScalarUDF / ScalarUDFImpl methods that are past the API health policy #24535.

Rationale for this change

The API health policy deprecation guidelines keep a deprecated API for 6 major versions or 6 months, whichever is longer. main is at 55.0.0, so anything deprecated in 50.0.0 or earlier can now be removed.

What changes are included in this PR?

The four methods named in the issue, all in datafusion/expr/src/udf.rs:

Item	Deprecated in
ScalarUDFImpl::is_nullable	45.0.0
ScalarUDF::display_name	50.0.0
ScalarUDF::is_nullable	50.0.0
ScalarUDFImpl::display_name	50.0.0

Removing them takes five more sites with them, which is worth spelling out since only one of the five is in the same file.

Two are the matching delegates on AliasedScalarUDFImpl, in udf.rs. That impl is annotated #[warn(clippy::missing_trait_methods)] and deliberately mirrors every ScalarUDFImpl method, so the pair has to go at the same time.

Three are display_name overrides in other crates, on ArrayElement and ArraySlice in datafusion/functions-nested/src/extract.rs, and on GetFieldFunc in datafusion/functions/src/core/getfield.rs. Rust fires the deprecation lint at call sites rather than at impl blocks, so these three carried no #[expect(deprecated)] marker and produced no warning before this change. Each of them also defines its own schema_name, which is the method that actually names output columns, so removing display_name changes no query output.

ExprSchema has no remaining use in udf.rs and comes out of the import list. That is the only added line in the diff; everything else is deletion.

No 56.0.0 upgrade guide note, per the issue.

Are these changes tested?

Covered by existing tests. The removed code was unreachable, so there is no new behaviour to add a test for.

Run locally on 1.97.0:

cargo fmt --all -- --check                                  clean
cargo clippy --all-targets --all-features -- -D warnings    clean
./dev/rust_lint.sh                                          clean (taplo, hawkeye, typos, prettier, rust_docs, workflow check)

Extended test suite, per AGENTS.md:

RUST_BACKTRACE=1 cargo test --profile ci \
    --exclude datafusion-examples --exclude datafusion-benchmarks --exclude datafusion-cli \
    --workspace --lib --tests --bins \
    --features avro,json,backtrace,extended_tests,recursive_protection,parquet_encryption

68 test binaries, 10702 passed, 0 failed, 8 ignored (the 8 are pre-existing #[ignore] tests, untouched here).

Are there any user-facing changes?

Yes. This removes public Rust API, so the api change label applies.

Downstream implementors of ScalarUDFImpl that override display_name or is_nullable will get a compile error naming the method. Callers of ScalarUDF::display_name or ScalarUDF::is_nullable were already getting a deprecation warning and will now get a compile error. is_nullable is replaced by return_field_from_args; display_name has no replacement because nothing in the codebase called it.