Makefile: remove inclusion which leads to confusing errors

This line gives some bad errors whenever ocamlfind is not in the
PATH or the visitors package is not installed, and may carry on
with the Makefile regardless of that failure (attempting to include
/Makefile.preprocess along the way).

	$ make
	make: ocamlfind: No such file or directory
	Makefile:2: /Makefile.preprocess: No such file or directory
	make: *** No rule to make target '/Makefile.preprocess'.  Stop.

It does not seem this inclusion is needed at all, so this patch removes
it. If is needed, it can be made safer like so:

	# make src/Ast.processed.ml
	_:=$(shell ocamlfind query)
	ifneq ($(.SHELLSTATUS),0)
	_: $(error "'ocamlfind query' failed, please install OCaml and put it in your PATH)
	endif
	visitors_root:=$(shell ocamlfind query visitors)
	ifneq ($(.SHELLSTATUS),0)
	_: $(error "'ocamlfind query visitors' failed, please 'opam install visitors')
	endif
	include $(visitors_root)/Makefile.preprocess

Makefile

@@ -1,6 +1,3 @@
-# make src/Ast.processed.ml
-include $(shell ocamlfind query visitors)/Makefile.preprocess
-
 .PHONY: all minimal clean test pre krmllib install
 
 ifeq ($(OS),Windows_NT)