Remove error messages from the event log

[rbev]

This pull request removes the event log entry for individual message failures from both the codebase and tests. Instead, failed messages are now only visible on the failed messages screen, not in the event log. This change reduces write and retention overhead on the event log's hot ingest path and simplifies related test logic.

Event log and message failure handling:

• Updated documentation in docs/eventlog-design.md to clarify that message failures are no longer recorded in the event log, but are instead visible only on the failed messages screen.
• Removed the MessageFailedDefinition class and its registration, which previously mapped message failure events to event log entries. [1] [2]

Test cleanup and simplification:

• Removed the test Should_add_an_event_log_item from When_a_message_has_failed.cs, which checked for event log entries related to message failures.
• Removed code and helper methods from When_hosting_error_ingestion_only.cs that waited for message failure event log entries, as these entries are no longer created. [1] [2]