p521: remove unneeded allow(dead_code) attribute #88

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	3m 9s
cross (armv7-unknown-linux-gnueabihf, 1.65.0)	4m 13s
build (1.65.0, wasm32-unknown-unknown)	4m 57s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 14s
build (stable, thumbv7em-none-eabi)	20s
doc	22s
cross (aarch64-unknown-linux-gnu, 1.65.0)	7m 41s
build (stable, wasm32-unknown-unknown)	14s
test (i686-unknown-linux-gnu, 1.65.0, sudo apt update && sudo apt install gcc-multilib)	6m 46s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 47s
cross (aarch64-unknown-linux-gnu, stable)	2m 5s
test (x86_64-unknown-linux-gnu, 1.65.0)	6m 50s
cross (powerpc-unknown-linux-gnu, 1.65.0)	4m 19s
test (x86_64-unknown-linux-gnu, stable)	1m 0s
cross (powerpc-unknown-linux-gnu, stable)	2m 27s
	47m 24s

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