Remove unused ArrayRef::is_arrow #5617

Job	Run time
Rust tests (tsan)	5m 51s
C++ API (coverage)	1m 10s
Rust/C++ FFI tests (asan)	2m 11s
Rust tests (miri)	1m 36s
Rust tests (asan)	6m 52s
Rust/C++ FFI tests (tsan)	1m 56s
Mirror DuckDB to R2 / Resolve DuckDB version and check R2	7s
Mirror DuckDB to R2 / Mirror DuckDB ${{ matrix.archive }} to R2	0s
DuckDB libraries available in R2	4s
Rust tests (coverage) (tests)	5m 49s
	25m 36s

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