[Comm-opaque2] Don't require mut self on register finish

Summary:
Small fixup, this was needed for opaque 1.2.0, but not
for opaque 2.0.

Test Plan:

cd shared/comm-opaque2/
cargo test

Depends on D7299

Reviewers: varun, bartek

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7362