concretizer: simplify "fact" method (#21148)
concretizer: simplify "fact" method (#21148)

The "fact" method before was dealing with multiple facts
registered per call, which was used when we were emitting
grounded rules from knowledge of the problem instance.

Now that the encoding is changed we can simplify the method
to deal only with a single fact per call.

(cherry picked from commit ba42c36f00fe40c047121a32117018eb93e0c4b1)

LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack