Don't provide RxNorm brand linkages for combo therapies

IE, if a Brand Name is associated with > 1 Ingredients, don't import it. We've been handling this downstream but it'd probably be easier to just avoid importing unusable brand IDs altogether.

Locate the RxNorm import entry point and the handling of brand-to-ingredient associations. Check how brands with multiple ingredients are currently represented, then verify that brands associated with more than one ingredient are excluded and that single-ingredient brands continue to import correctly.

Written by the indexing model from the issue text.