chore: remove a stabilized impl_core function #2038

Job	Run time
clippy	1m 18s
docs	52s
check no_std	38s
features stable	1m 39s
fmt	21s
features stable --all-targets	2m 38s
miri --no-default-features	5m 13s
features nightly	2m 27s
test ubuntu-latest stable --no-default-features	1m 6s
miri	5m 36s
features nightly --all-targets	3m 39s
test ubuntu-latest stable	1m 0s
wasm	27s
miri --all-features	7m 25s
test ubuntu-latest nightly --no-default-features	1m 20s
test ubuntu-latest nightly	1m 15s
test ubuntu-latest 1.79 --no-default-features	31s
test ubuntu-latest 1.79	31s
test windows-latest stable --no-default-features	4m 19s
test windows-latest stable	2m 52s
test windows-latest nightly --no-default-features	3m 49s
test windows-latest nightly	2m 52s
test windows-latest 1.79 --no-default-features	1m 9s
test windows-latest 1.79	1m 9s
test ubuntu-latest 1.79 --features json	33s
test windows-latest 1.79 --features json	1m 17s
test ubuntu-latest nightly --all-features	1m 28s
test windows-latest nightly --all-features	3m 17s
	1h 0m 41s

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