Retire the tcp-chain static test topology

The tcp-chain profile has never been runnable. ping-test.sh dispatches on
chain and mesh only, and the string tcp-chain has never appeared in that
script in its history, so invoking the profile has always fallen through
to the unknown-profile branch: silently asserting nothing before that
branch was made to fail, and exiting 2 since. Neither runner referenced
it either.

What it would have covered is already covered. The chaos tcp-mesh
scenario runs in local CI and gives TCP a discriminating gate: n04's only
edges are TCP, so if the transport were broken n04 could not parent and
the scenario's baseline assertion would fail. It also carries a pure-TCP
two-hop path, n01 to n04 to n05, under netem and link flaps. The only
residual tcp-chain would have added is a mesh with no UDP present
anywhere, which is not worth a fixture that has never run.

Remove the topology, its three compose services, and the documentation
rows. Every profile the static compose file still defines is now
exercised by a suite.

Also correct the chaos README, which has listed tcp-only and tcp-chain in
its transport table since those two scenarios were deleted in 9e63b42.
Neither has existed for months.

testing/README.md

@@ -16,7 +16,6 @@ configurations.
 | ----------- | ----- | --------- | -------------------------------- |
 | mesh        | 5     | UDP       | Sparse mesh, 6 links, multi-hop  |
 | chain       | 5     | UDP       | Linear chain, max 4-hop paths    |
-| tcp-chain   | 3     | TCP       | Linear chain over TCP (port 8443) |
 | rekey       | 5     | UDP       | Rekey integration test topology  |
 
 ### [tor/](tor/) -- Tor Transport Integration