Remove optional prev flag #23

Job	Run time
rust-check-git-rev-deps	19s
cargo-deny (advisories)	1m 0s
cackle	1m 22s
cargo-deny (bans licenses sources)	50s
build (gcc, current)	2m 38s
fmt	18s
build (gcc, current, --check-test-tx-meta)	2m 28s
build (gcc, next)	3m 7s
build (gcc, next, --check-test-tx-meta)	2m 31s
build (clang, current)	2m 36s
build (clang, current, --check-test-tx-meta)	2m 33s
build (clang, next)	2m 31s
build (clang, next, --check-test-tx-meta)	2m 35s
complete	1s
	24m 49s

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