[services][blob] Remove old C++ implementation

Summary:
Removed the services/blob/old directory containing C++ impl

Resolves ENG-2306

Test Plan:

• CI should not depend on this anymore.
• Running and building the service locally runs Rust implementation

Reviewers: tomek, jon, ashoat, varun

Reviewed By: ashoat

Subscribers: atul

Differential Revision: https://phab.comm.dev/D5768