Drop dead libraries flagged by monopam Dead_lib across the repo

Remove libraries declared in '(libraries ...)' clauses but unreferenced
by any module in the same source tree, as flagged by 'monopam lint'
after the new Dead_lib detection landed. Touches 131 dune files across
~80 packages.

A few stanzas needed a positive correction instead of a pure removal:

- ocaml-git/bin/diag: depended on eio_main + bytesrw-eio for an
Eio_posix.run call site; the umbrella was overkill, switch to the
precise eio_posix package.
- ocaml-scaleway/lib, ocaml-s3/lib: scaleway.mli / s3.mli reference
Eio_unix.Stdenv.base; eio.unix is required and was missing.
- merlint/lib: pulled bytesrw + nox-opam.bytesrw to surface
Opam_bytesrw, used by rule e915 and lint helpers.

Stanzas where Dead_lib was a false positive (transitive dep needed
for module visibility, virtual-library impls) are left untouched —
e.g. helix.jx.jsoo for ocaml-globe/demo retains its (libraries ...)
entry because it provides the impl of the helix.jx virtual lib.