Revert "update ocaml-sha to use dune instead of jbuilder"

This reverts commit ba02f7dbe21fa8fe94cb1ae61a903c8fb6beb03e.
It used dune 2.0 which isn't supported on OpenBSD at this point.

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
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# On branch master
# Your branch is up to date with 'origin/master'.
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# Changes to be committed:
#	modified:   VERSIONS
#	modified:   ocaml-sha/Makefile
#	modified:   ocaml-sha/bitfn.h
#	deleted:    ocaml-sha/c_flags.ml
#	deleted:    ocaml-sha/dune
#	deleted:    ocaml-sha/dune-project
#	new file:   ocaml-sha/jbuild
#	modified:   ocaml-sha/sha.opam
#	modified:   ocaml-sha/sha1.h
#	modified:   ocaml-sha/sha1_stubs.c
#	new file:   ocaml-sha/sha1c.c
#	modified:   ocaml-sha/sha256.h
#	modified:   ocaml-sha/sha256_stubs.c
#	new file:   ocaml-sha/sha256c.c
#	modified:   ocaml-sha/sha512.h
#	modified:   ocaml-sha/sha512_stubs.c
#	new file:   ocaml-sha/sha512c.c
#	deleted:    ocaml-sha/test/dune
#	new file:   ocaml-sha/test/jbuild
#
