Stop auto-wiring piggieback via :requires magic

[bbatsov]

Right now refactor-nrepl.middleware/wrap-refactor declares #'cider.piggieback/wrap-cljs-repl in its descriptor's :requires set whenever piggieback happens to be on the classpath. nREPL responds to this by warning that piggieback isn't in the middleware list and then injecting it automatically (extended-descriptors in nrepl.middleware, marked TODO: stop adding missing deps in future versions.).

This is exactly the kind of magic that cider-nrepl 0.59 set out to remove with PR #971 — they deprecated their own requires-piggieback helper to a no-op for the same reason. We're currently leaning on the deprecated nREPL behavior instead, just inlined locally.

Cleaner direction:

1. Delete maybe-require-piggieback and don't put piggieback into :requires. The wrap-refactor descriptor stays piggieback-free.
2. The only test that needs cljs is refactor-nrepl.ns.resolve-missing-test. Its fixture already passes piggieback to default-handler, but the rebinding happens after session-fixture has already started the server, so the binding doesn't take effect. The fixture currently "works" only because nREPL auto-injects piggieback. Restructure it to start the server inside the binding (or stop composing with the default session-fixture for this test).
3. Optional: inline cider.nrepl.middleware.util.cljs/grab-cljs-env inside refactor-nrepl.ns.resolve-missing so we no longer depend on cider-nrepl's util/cljs namespace at all.

Not urgent — the current setup works on cider-nrepl 0.59 and the test passes locally and in CI. But extended-descriptors is on borrowed time so it's worth doing before nREPL drops it.