fix: remove duplicate F64x8 assign op impls #77

Job	Run time
Pass MSRV values to other jobs	4s
clippy/stable	22s
format/stable	25s
cross_test/s390x-unknown-linux-gnu/stable	1m 36s
nostd/thumbv6m-none-eabi/stable	27s
cross_test/i686-unknown-linux-gnu/stable	1m 34s
native-backend/stable	38s
miri	0s
cargo-careful	0s
docs/nightly	0s
tests/beta	1m 25s
tests/1.64.0	1m 24s
tests/stable	1m 27s
blas-msrv	49s
conclusion	0s
	10m 11s

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