Scrub dangling Janet references; drop dead Janet-coupled files

Rephrase comments that pointed at deleted Janet files (emit.janet, the seed
sources, 'the Janet back end punts ...') to present-tense descriptions of the
Chez behavior. Comment/docstring-only; the self-host fixpoint is unchanged
(comments don't affect the compiled seed).

Delete five files that were Janet-host shims with no Chez path: clojure.java.io
(provided natively by host/chez/io.ss), and jolt.{nrepl,png,interop,shell}
(the janet.* bridge, os/shell, janet.net — none exist on Chez).

jolt-cf1q.6

host/chez/collections.ss

@@ -205,7 +205,7 @@
         ((pset? coll) (pset-conj coll x))
         ;; a list/seq conjs by PREPENDING (seq.ss: cseq / empty-list). conj onto a
         ;; list stays a list, conj onto a lazy/realized seq yields a seq cell (a
-        ;; Cons) — list?-preserving, matching the seed.
+        ;; Cons) — list?-preserving.
         ((cseq? coll) (if (cseq-list? coll) (cseq-list x coll) (cseq-realized x coll)))
         ((empty-list-t? coll) (cseq-list x jolt-nil))
         ((pmap? coll)