pf: clear PF_TAG_ROUTE_TO for dummynet fast path

Similar to the PF_TAG_DUMMYNET we must also clear the route tag if
dummynet didn't keep the packet. In that case we'd continue immediately
and there'd be no need for the route tag. Keeping it could lead to
unexpected routing of traffic.

See also: 27407a6adc793bdfaef8a86ece32fb1b461429f0
See also: https://redmine.pfsense.org/issues/14055
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