Fix Xcode 16-beta2 duplicate file warning

[scannillo]

Summary

Running our Demo app with XCode 16-beta2 shows the following warning:

Changes

• Remove duplicate Demo pbxproj entry for BTSEPADirectDebitTestHelper.swift

Checklist

• ~ Added a changelog entry~

Authors

@scannillo

[jaxdesmarais]

Fwiw, we also have this warning on Xcode 15, so resolves the warning across the board! 🥳