SetoidRewrite: various cleanups and simplifications #3900

Job	Run time
opam-build (latest, ubuntu-latest)	4m 37s
opam-build (dev, ubuntu-latest)	4m 7s
quick-build (supported, ubuntu-latest)	3m 34s
nix	2m 14s
build (supported)	3m 31s
opam-build (supported, ubuntu-latest)	4m 45s
build (latest)	3m 40s
quick-build (latest, ubuntu-latest)	3m 16s
quick-build (dev, ubuntu-latest)	3m 15s
build (dev, --warnings)	3m 55s
doc-alectryon	15m 34s
doc-dep-graphs	7m 34s
doc-coqdoc	1m 46s
coqchk (supported)	2m 8s
install (dev)	1m 52s
doc-timing	2m 0s
coqchk (latest)	2m 8s
coqchk (dev)	2m 6s
install (latest)	1m 30s
install (supported)	1m 47s
delete-artifacts	7s
deploy-doc	0s
	1h 15m 26s

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