[reductions] Delete another piece of dead flag handling [NFC]

The code claimed to handle nsw/nuw, but those aren't passed via builder state and the explicit IR construction just above never sets them.

The only case this bit of code is actually relevant for is FMF flags.  However, dropPoisonGeneratingFlags currently doesn't know about FMF at all, so this was a noop.  It's also unneeded, as the caller explicitly configures the flags on the builder before this call, and the flags on the individual ops should be controled by the intrinsic flags anyways.  If any of the flags aren't safe to propagate, the caller needs to make that change.