`fn small_memset`: Remove the unused `UP_TO == 64` case (#1314) #877

Job	Run time
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release --no-default-features --featu...	11m 49s
test-on-ubuntu-latest (x86_64-unknown-linux-gnu, release, --release)	14m 4s
	25m 53s

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