[lib] Remove getCurrentUser

Summary:
We needed getCurrentUser in order to "patch" permissions based on block. However, we're now considering filtered permissions as part of useThreadsWithPermission so we can handle blocks as part of permission check vs. RawThreadInfo -> ThreadInfo. This greatly simplifies deprecation of memberHasAdminPowers on the client.

Depends on D12154

Test Plan: flow + ensuring that useExistingThreadInfoFinder, threadInfoFromRawThreadInfo, etc. continue to work as expected.

Reviewers: ashoat, ginsu, tomek, varun

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D12158