async-signature: remove `AsyncDigestSigner` blanket implementation #2097

Job	Run time
test (1.75.0)	21s
minimal-versions	0s
no_std (1.75.0, thumbv7em-none-eabi)	11s
test (stable)	14s
no_std (1.75.0, wasm32-unknown-unknown)	18s
no_std (stable, thumbv7em-none-eabi)	6s
no_std (stable, wasm32-unknown-unknown)	18s
	1m 28s

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