This commit is a collection of various minor
touchups extracted from multiple iterations of
MDEV-34614 mysqlbinlog warn on EOF before GTID in --stop-position.

* `rpl_gtid.h`: Remove unimplemented function prototype
  `Window_gtid_event_filter::verify_gtid_is_expected`
  (it’s also a red herring to MDEV-34614)
* `rpl_gtid.cc`: Add brief description to `report_audit_findings`
* `binlog_mysqlbinlog_warn_stop_position`: Name the temporary output
  redirect as ignored to match `binlog_mysqlbinlog_warn_stop_datetime`
  * #3400 (comment)
* `binlog_mysqlbinlog_warn_stop_datetime`: Remove duplicate case

This commit adds warnings for `--stop-position` GTIDs
that were not reached at EOF, mainly as a follow-up to
MDEV-27037 Mysqlbinlog should output a warning if EOF is found before its stop condition
(#3400).

`--stop-position` warnings inform possible mistakes in the input,
especially for progress reporting of scripts/wrappers.
MDEV-34614 enhances MDEV-27037 with individualized GTID validation, for
GTID range selection weren’t in all versions that MDEV-27037 targeted.

The `Gtid_event_filter` family provides the the warning
mechanism polymorphically and through the new public method
`verify_completed_state`. It’s uncommon to commit a new public
API targeting a non-latest version, but this design is not only
hierarchical but also extensible (e.g., to `--ignore-server-ids`).