Revert "[keyserver] implement registerUser rpc with napi-rs/tonic/opaque-ke"

Summary: This reverts commit df8ac1ace8d3b0a7cac123a1828e788ceedd5f83.

Test Plan: reverting a change, shouldn't break anything

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6411