Makefile: Don't overwrite tokio_unstable RUSTFLAGS value

Don't allow the Makefile to overwrite a necessary RUSTFLAGS value to use
tokio_unstable: '--cfg tokio_unstable'.

Makefile

@@ -5,8 +5,6 @@ prog :=neptune-core
 debug ?=
 
 $(info debug is $(debug))
-# Treat all warnings as errors
-export RUSTFLAGS = -Dwarnings
 
 # Set another target dir than default to avoid builds from `make`
 # to invalidate cache from barebones use of `cargo` commands.