arena_tree: remove unused allow(dead_code). #953

Job	Run time
build_wasm (nightly)	49s
build_lib_test (nightly)	49s
build_wasm (beta)	49s
no_features_build_test (nightly)	27s
build_lib_test (beta)	1m 16s
build_bin_spec (nightly)	1m 7s
build_wasm (stable)	40s
no_features_build_test (beta)	33s
build_lib_test (stable)	1m 4s
build_bin_spec (beta)	1m 10s
build_wasm ($MSRV)	2m 48s
no_features_build_test (stable)	22s
build_lib_test ($MSRV)	3m 16s
build_bin_spec (stable)	1m 25s
no_features_build_test ($MSRV)	2m 19s
build_bin_spec ($MSRV)	2m 57s
clippy_format	2m 14s
lockfile	58s
	25m 3s

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