[services] Delete base-image and sandbox mode
Summary:
No services use the C++ base-image anymore so it can be deleted. Also, removed leftovers of sandbox mode, which we decided to remove some time ago.
Test Plan: None, this is just a cleanup. Did a git grep though to make sure nothing is using it.
Reviewers: michal, varun, kamil, jon
Reviewed By: michal
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D9137