docs: reflect that the Hetzner runner was torn down; drop the smoke-test workflow

Only the maintainer's Windows machine registration remains. Notes that
adding a second host back is what restores per-leg parallelism for a
matrix job, without needing any other change.

Also removes self-hosted-smoke.yml: linux-compilers, linux-sanitizers,
and linux-all-features now exercise the self-hosted runner directly on
every real CI run, making the standalone smoke test redundant.

Author: Yaraslau Tamashevich

.github/self-hosted-runner/README.md

@@ -6,15 +6,15 @@ a Docker container. Used by `.github/workflows/self-hosted-smoke.yml`
 `linux-compilers` job whenever one is online and idle (see **ci.yml
 integration** below).
 
-Currently running on two hosts, each a separate registration of the same
-image:
-
-- the maintainer's Windows machine (Docker Desktop, Linux containers)
-- a Hetzner Cloud VM (`morph-hetzner-hel1`)
-
-Having more than one matters for `linux-compilers`: it's a 4-leg matrix,
-and a single runner can only execute one job at a time, so with just one
-box the legs queue behind each other instead of running in parallel.
+Currently running on one host: the maintainer's Windows machine (Docker
+Desktop, Linux containers). A second registration briefly ran on a
+Hetzner Cloud VM (`morph-hetzner-hel1`) but that machine was torn down.
+
+A single runner can only execute one job at a time, so with just one box
+a matrix's legs queue behind each other on it instead of running in
+parallel the way separate GitHub-hosted VMs would. Adding a second host
+back (see **Running more than one runner** below) restores that
+parallelism; nothing else needs to change to support it.
 
 The image is plain Ubuntu 24.04 with just the runner binary and enough
 packages (`sudo`, `curl`, `git`, build-essential-adjacent tooling) to