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This PR removes the remove: field from the 74 packages where it only deleted what the package had installed under the switch: the 73 ocamlfind -toolchain android remove <pkg> lines and cmdliner-android's rm -rf of its own library directory.

opam 2.6 cannot rebuild a package that has a remove field. The build directory is left without its sources, so the first build command that touches a source file fails. Any upgrade of a native dependency triggers it on a switch that already holds the package, which is how it showed up: ppxlib 0.39.0 was published, opam upgrade wanted to recompile ppx_deriving-android, and its build stopped at once.

sh -c ": > src/dune"
cannot create src/dune: Directory nonexistent

It reproduces without any cross-compilation, with a two-package repository where b depends on a, has a source archive and a remove field: install a.1 and b, publish a.2, run opam upgrade. opam 2.5.2 rebuilds b; 2.6.0 and 2.6.1 fail as above, whether the remove script succeeds or not, and whatever it runs (ocamlfind remove, rm -rf). Without the field, 2.6.1 rebuilds b, and opam remove b still deletes its files, since opam tracks what an install adds to the switch.

The ocamlfind remove lines were also failing by themselves on every package that installs a sub-directory (ocamlfind: …/lib/ppx_deriving/create: Is a directory), which opam reports as a warning and carries on from.

Four packages keep their script and will hit the same failure when opam 2.6 rebuilds them: ocaml-android (remove.sh), build-android-sysroot, build-gmp-android and build-libev-android. I left them alone because I could not check that opam's own tracking covers everything they install.

Not tested here: the cross builds themselves. The check was the reproduction above and opam lint on the edited files.