chore: remove dead indexed_field.rs

The `pub mod indexed_field;` declaration and the PyGetIndexedField class
registration were dropped from crates/core/src/expr.rs in b5446ef (#728)
when DataFusion 39 replaced Expr::GetIndexField with the FieldAccessor
trait, but the file stayed on disk. It has not been compiled since, and it
imports GetIndexedField, which no longer exists upstream.

The nightly pre-commit rust-fmt hook formats files by path, so it keeps
rewriting the orphan; stable `cargo fmt --check` in CI walks the module
tree and never sees it. Deleting the file ends that churn.

Closes #1745

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: timsaucer