Prepare to import ssrmatching in 8.9.0

Upstream has corrected most of the licensing issues with ssrmatching
in 8.9.0. That version introduced one new file with a CeCILL-B license,
but it’s an .mli file, so OCaml should be able to infer its contents.
Don’t import the offending .mli, but do import the files with corrected
license headers. Remove the patch introduced in
5d3dc22c (which backported the
correctly licensed files).