chore(quic-interop): drop the smoke target — runner is the canonical path

make smoke was the bisector for "is the bug in :quic or in the runner
environment?" while we were debugging the initial close-path / PTO /
padding issues. Now that the runner reliably runs the matrix end-to-end
(handshake + chacha20 green vs aioquic), smoke's only job — running
picoquic outside the runner — is unneeded, and the picoquic image
keeps changing its entrypoint / required args in ways that make smoke
finicky to maintain.

Removes:
  - make smoke / smoke-down targets
  - SMOKE_NET / SMOKE_PICOQUIC / SMOKE_CLIENT vars
  - SMOKE_MODE handling in run_endpoint.sh (just always tolerates
    /setup.sh failure now — same effect, less ceremony)
  - Plan doc note about make smoke updated to reflect removal.

If we ever need a non-runner bisector again, it's two `docker run`
commands; not worth permanent maintenance.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

quic/interop/plans/2026-05-06-interop-runner.md

@@ -22,7 +22,10 @@ reorder / migration scenarios that are awkward to reproduce in unit tests.
   OpenJDK 21 runtime, copies the `installDist` output.
 - `run_endpoint.sh` sources the base image's `/setup.sh` then execs our JVM
   binary.
-- `Makefile` wrappers: `make build`, `make smoke`, `make clean`.
+- `Makefile` wrappers: `make build`, `make clean`. (A `make smoke`
+  target previously stood up picoquic + our endpoint on a private Docker
+  bridge to bisect runner failures from impl failures; dropped once
+  the runner reliably exercised both paths.)
 
 ## Local iteration loop