Revert use of getUniqueUndroppableUser in AssumeBundleBuilder

Fix build bot failure in rG4ac4e521 caused due to assumeBundleBuilder
using new API (getUniqueUndroppableUser).
We now continue using the existing API for AssumeBundleBuilder
(getSingleUndroppableUser).

Sorry for the noise here.

Tests-Run: failing testcase passes.