Remove dead cases in utf8lwrcodepoint() and utf8uprcodepoint() #20

Job	Run time
build (ubuntu-latest, Debug, address)	17s
build (ubuntu-latest, Debug, memory)	34s
build (ubuntu-latest, Debug, undefined)	47s
build (ubuntu-latest, RelWithDebInfo, address)	26s
build (ubuntu-latest, RelWithDebInfo, memory)	19s
build (ubuntu-latest, RelWithDebInfo, undefined)	23s
build (ubuntu-latest, MinSizeRel, address)	19s
build (ubuntu-latest, MinSizeRel, memory)	14s
build (ubuntu-latest, MinSizeRel, undefined)	19s
build (ubuntu-latest, Release, address)	37s
build (ubuntu-latest, Release, memory)	21s
build (ubuntu-latest, Release, undefined)	23s
build (macOS-latest, Debug, address)	15s
build (macOS-latest, Debug, undefined)	18s
build (macOS-latest, RelWithDebInfo, address)	41s
build (macOS-latest, RelWithDebInfo, undefined)	21s
build (macOS-latest, MinSizeRel, address)	20s
build (macOS-latest, MinSizeRel, undefined)	39s
build (macOS-latest, Release, address)	18s
build (macOS-latest, Release, undefined)	25s
	8m 16s

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