[9.18] fix: usr: Clear REDIRECT flag when it isn't needed

When `nxdomain-redirect` is in use, and a recursive query is used to get the redirected answer, a flag is set to distinguish it from a normal recursive response. Previously, that flag was left set afterward, which could trigger an assertion if a normal recursive query was sent later on behalf of the same client: for example, because the `filter-aaaa` plugin was in use.  This has been fixed.

Closes #5936

Backport of MR !12073

Merge branch 'backport-5936-clear-redirect-flag-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!12077