services: Remove gRPC source after installing

Summary:
This reduces size of the Docker base image from 3.03GB to 1.29GB.

Also:

services: Pass gRPC build flags at build time
services: Better Docker layering in base image
services: Remove unused gRPC build variable

Test Plan: Run npm run build-all and run-all

Reviewers: karol-bisztyga, ashoat, geekbrother

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, atul, benschac

Differential Revision: https://phabricator.ashoat.com/D3399