test: delete sequential/test-debug-cluster

The test does not work well with concurrent invocations of the test
runner because it uses fixed port numbers.  The functionality it tests
is covered by sequential/test-debug-port-cluster, a verbatim copy with
the only difference being that it doesn't use fixed port numbers.

PR-URL: #306
Reviewed-By: Miroslav Bajtoš <[email protected]>