[keyserver] Stop returning set of deviceTokens in rescindPushNotifs(...)

Summary:
We were previously using the set of deviceTokens returned by rescindPushNotifs(...) in rescindAndUpdateBadgeCounts to skip updateBadgeCount(...) for devices that had notifications rescinded. See D5606 for context.

Basically, we don't need the set of deviceTokens in rescindAndUpdateBadgeCounts and it isn't used anywhere else so it can be removed.

Depends on D5606

Test Plan:
Looked at other usages of rescindPushNotifs(...) and the return value was never used:

Reviewers: tomek, marcin, varun, ginsu, rohan, ashoat

Reviewed By: ashoat

Subscribers: ashoat, abosh

Differential Revision: https://phab.comm.dev/D5607