[lib] Avoid unnecessary Set construction in RadixTree.getAllMatchingExactly

Summary:
Noticed this after landing the diff here. The underlying data is a Set anyways, so all this does it copy the Set. We don't return the original Set anyways, so there is no point to this.

Depends on D9646

Test Plan: Flow, careful code inspection

Reviewers: tomek, atul, inka, rohan

Reviewed By: atul

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9647