Phase 0: Disable the relay_sys.c entry point in --disable-module-relay

When compiling with --disable-module-relay, we should disable the relay_sys.c entry point.

At each stage, we should work to minimize layer-violations: there should generally not be calls from src/core/ into relay-specific code, and we should plan to refactor as needed to minimize them. We can reduce layer violations in parallel with the above.