Remove redundant result of `AggregateFunctionExpr::field` #31544

Job	Run time
cargo check	6m 22s
Check cargo fmt	35s
build with wasm-pack	2m 32s
cargo test (macos)	9m 14s
cargo test (macos-aarch64)	10m 13s
Verify MSRV (Min Supported Rust Version)	7m 30s
Verify Vendored Code	55s
cargo test (win64)	44m 13s
cargo test datafusion-cli (amd64)	4m 1s
cargo examples (amd64)	13m 36s
cargo test (amd64)	9m 31s
check Cargo.toml formatting	2m 50s
clippy	5m 26s
check configs.md is up-to-date	2m 57s
cargo test hash collisions (amd64)	14m 25s
cargo doc	4m 10s
cargo test doc (amd64)	14m 21s
Run sqllogictest with Postgres runner	3m 34s
cargo test pyarrow (amd64)	3m 1s
verify benchmark results (amd64)	12m 6s
	2h 51m 32s

Provide feedback