[nix] remove unused nix files

Summary:
we no longer need v3.15.8 of protobuf. we should use v3.21.12 everywhere.

additionally, we don't seem to use blob.nix anywhere. it's been broken for months since 3.15.8 was removed from the nix dev environment in D8843, so i think it's safe to remove entirely.

note that protobuf changed their branch naming so v21.12 is actually semver 3.21.12

Test Plan: android CI passes. services docker build CIs pass.

Reviewers: atul, will, bartek, O1 Nix

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11189