Drop Makefile targets aside from bench

Only the `bench` target is really used (by current-bench).

Author: polytypic

Makefile

@@ -1,15 +1,4 @@
-.PHONY: build test bench
-
-all: build test bench
-
-build:
-	@dune build @install
-
-test:
-	@dune runtest --force
+.PHONY: bench
 
 bench:
 	@dune exec --release -- bench/main.exe -budget 1
-
-clean:
-	@dune clean