ecdsa

ecdsa: simplify `serde` feature #1731

Job	Run time
test (1.81.0)	30s
no_std (thumbv7em-none-eabi, 1.81.0)	1m 59s
test (stable)	30s
no_std (thumbv7em-none-eabi, stable)	1m 52s
no_std (wasm32-unknown-unknown, 1.81.0)	1m 51s
no_std (wasm32-unknown-unknown, stable)	1m 50s
	8m 32s

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