[identity] remove getUserPublicKey RPC

Summary: we are going to replace this later in the stack with a GetSessionInitializationInfo RPC. for now, just removing without replacing so I can put up another diff that removes the deviceID concept without modifying soon-to-be dead code.

Test Plan: cargo build, nothing calls this code currently

Reviewers: jon, tomek, bartek

Reviewed By: jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6839