Delete duplicate broken orphan modules

These three modules were committed in 8b8fc03 ("further proofing bridges")
but never compiled and duplicated functionality already covered by
in-suite modules:

* EchoCategory.agda (58 sigs, 0 data) — duplicate of EchoCategorical.agda;
  had self-shadowing `_→_` and `_∘_` definitions that parse-errored.

* EchoCNO.agda (8 sigs) — duplicate of EchoCNOBridge.agda; used invalid
  `Σ (x : A) (λ _ → ...)` binder syntax and had a dangling top-level
  `where` clause.

* EchoJanusSimple.agda (9 sigs) — duplicate of EchoJanusBridge.agda;
  same invalid `Σ (x : A) (...)` binder syntax.

Nothing in the verified suite (All.agda) imported any of them. Deleting
them removes silently-broken Agda from the tree without changing what
compiles.

https://claude.ai/code/session_01JRLz84fAaWvRBKyXuc4tyK

Author: claude