[lib] Deprecate userSearchIndexForPotentialMembers and userStoreSearchIndex

Summary:
Now that the remaining search experiences have been updated, we can safely deprecate both userSearchIndexForPotentialMembers and userStoreSearchIndex since there are no usages in the codebase.

Addresses ENG-5435

Depends on D10253

Test Plan: Ran flow and made sure everything on the app still works fine

Reviewers: atul, ginsu, inka

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10254