ml-dsa: remove `std` feature (#893) #177

Job	Run time
test (1.81.0)	7m 44s
no_std (thumbv7em-none-eabi, 1.81.0)	20s
test (stable)	7m 32s
no_std (thumbv7em-none-eabi, stable)	18s
no_std (wasm32-unknown-unknown, 1.81.0)	20s
no_std (wasm32-unknown-unknown, stable)	16s
	16m 30s

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