Modeled a mesh grille but forgot to clean up these and now I don't want to remodel it all over again
As far as I get it you do want to remove "interior faces" and they're repetitive and you don't want to do it one by one, so here is a replicate of your issue down here in this animated image.
- Hit TAB key on your keyboard to get into Edit Mode
- From the select menu choose "Select All by Trait" then choose "Interior Faces"
- Press X and delete those faces