Remove findlib files

[whitequark]

See ocaml/opam#2948.

[avsm]

@dsheets @yallop @AltGr @samoht - could you please LGTM if its ok?

[camelus]

✅ No new or changed opam files c6f4a3d

---

✅ Installability check (6710 → 6710)

[yallop]

LGTM

$ git diff --summary HEAD~..HEAD | wc -l 
3115
$ git diff --summary HEAD~..HEAD | grep -c  '^ delete .*/findlib'
3115
$ find . -name findlib
$