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

Merge branch '5936-clear-redirect-flag' into 'main'

See merge request isc-projects/bind9!12073