Remove now-unused sync types #8749

Job	Run time
ormolu	9s
build ucm (ubuntu-20.04)	2m 44s
build ucm (macOS-12)	6m 4s
build ucm (windows-2019)	8m 50s
generate jit source	10s
run interpreter tests (ubuntu-20.04)	1m 6s
run transcripts (ubuntu-20.04)	3m 44s
run interpreter tests (macOS-12)	1m 48s
run transcripts (macOS-12)	7m 56s
run transcripts (windows-2019)	6m 32s
build jit binary / build jit binary (ubuntu-20.04)	7s
build jit binary / build jit binary (macOS-12)	16s
build jit binary / build jit binary (windows-2019)	17s
test jit / test jit (ubuntu-20.04)	13s
test jit / test jit (macOS-12)	13s
	40m 9s

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