remove reserved wallet registration RPC
Summary:
we don't need this. SIWE is enough to prove that a user owns a particular wallet address
Depends on D12546
Test Plan: everything builds, git grep'd for any remaining references to this RPC and message type
Reviewers: bartek
Reviewed By: bartek
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D12547