[chore] remove unused files and folders
Summary:
removed comm-opaque and all references to it
removed shared/protos/_generated since we don't use any generated C++ files for gRPC anymore
removed opaque-ke from the identity service since we use comm-opaque2 instead
Test Plan: built keyserver and identity service successfully. will see if CI passes
Reviewers: atul, jon, ashoat
Reviewed By: atul, jon, ashoat
Subscribers: tomek
Differential Revision: https://phab.comm.dev/D8334