cleanup in FinNat #3020

Job	Run time
build (supported)	3m 29s
quick-build (supported, ubuntu-latest)	3m 18s
nix	2m 28s
build (latest)	3m 28s
quick-build (latest, ubuntu-latest)	3m 10s
opam-build (supported, ubuntu-latest)	4m 50s
quick-build (dev, ubuntu-latest)	3m 11s
build (dev, --warnings)	3m 17s
opam-build (latest, ubuntu-latest)	4m 41s
opam-build (dev, ubuntu-latest)	4m 27s
doc-alectryon	15m 27s
doc-coqdoc	1m 28s
coqchk (supported)	2m 9s
doc-timing	1m 31s
coqchk (latest)	2m 6s
install (supported)	1m 36s
coqchk (dev)	1m 45s
doc-dep-graphs	7m 35s
install (latest)	1m 32s
install (dev)	1m 37s
deploy-doc	0s
delete-artifacts	9s
	1h 13m 14s

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