[blob][backup] Remove unused build dependency

Summary:
Noticed that this is a leftover from the time when we used to build protos
directly in services.
Now blob and backup no longer expose gRPC API and use the grpc-clients crate to call Identity service.

Test Plan: cargo build for both services.

Reviewers: michal, varun

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10130