Remove now-unused sync types #8750

Job	Run time
build ucm (ubuntu-20.04)	3m 13s
ormolu	14s
build ucm (macOS-12)	6m 32s
build ucm (windows-2019)	5m 48s
generate jit source	10s
run transcripts (ubuntu-20.04)	3m 44s
run interpreter tests (ubuntu-20.04)	1m 3s
run transcripts (macOS-12)	6m 51s
run interpreter tests (macOS-12)	2m 2s
run transcripts (windows-2019)	6m 13s
build jit binary / build jit binary (ubuntu-20.04)	14s
build jit binary / build jit binary (macOS-12)	19s
build jit binary / build jit binary (windows-2019)	20s
test jit / test jit (ubuntu-20.04)	6s
test jit / test jit (macOS-12)	17s
	37m 6s

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