Remove external server setup from CI and tests

The tests that required an externally running QuestDB instance have
been migrated to the core QuestDB module, where they run against an
embedded server. The infrastructure that supported the old approach
is no longer needed here.

Deleted files:
- AbstractQdbTest and AbstractLineSenderTest: dead test base classes
  with no remaining subclasses or callers.
- ci/questdb_start.yaml, ci/questdb_stop.yaml: scripts that started
  and stopped a QuestDB process around each test run.
- ci/confs/: default and authenticated server.conf files (and the
  authDb.txt credential file) used to configure those server
  instances.

Updated files:
- ci/run_client_tests.yaml: removed the configPath parameter and the
  start/stop template calls; the step now just runs the Maven tests.
- ci/run_tests_pipeline.yaml: removed QUESTDB_RUNNING and
  QUESTDB_ILP_TCP_AUTH_ENABLE variables, collapsed the two
  run_client_tests.yaml invocations (default + authenticated) into
  one, and removed the "Enable ILP TCP Auth" step between them.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mtopolnik