You are right, 7.1.5 don't have the option to Remove branch ruleset from application rule. I too tried in 7.1.9 to remove the branch ruleset on application rule from using "Remove from application" and save. This ruleset disappears from the branch, but on rule-form refresh the ruleset comes back in.
For me this appears to be a bug, there should be some explanation on why the ruleset comes back in. However, you can log one SR stating this as "Product Defect" as there is an option to remove branch ruleset in designer studio.