elliptic-curve: remove `alloc` from `serde` feature #3462

Job	Run time
build (stable, thumbv7em-none-eabi)	1m 8s
build (1.85.0, wasm32-unknown-unknown)	1m 9s
test-careful	1m 5s
test-miri	48s
test (1.85.0)	39s
build (1.85.0, thumbv7em-none-eabi)	1m 8s
test (stable)	30s
build (stable, wasm32-unknown-unknown)	1m 4s
test (nightly)	33s
minimal-versions	0s
	8m 4s

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