Makefile.monolith: remove [dune build @check].

This was useless and did not use the correct switch.

Makefile.monolith

@@ -62,7 +62,6 @@ BUILD := $(shell \
 
 .PHONY: all
 all:
-	@ $(DUNEBUILD) @check # build src/.merlin, etc.
 	@(echo "(lang dune 2.0)" && \
 	  echo "(context (opam (switch $(SWITCH))))" \
 	 ) > dune-workspace.afl