[minor] Makefile.menhir bugfix for non-subsecond make systems

François Pottier reported that, on his mac machine,
"make promote-menhir; make world" fails: it seemed
that parsing/parser.ml, after being produced as a temporary
file by the promote-menhir target, is not refreshed using
the `parsing/parser.ml: boot/menhir/parser.ml` rule from
Makefile (which does the MenhirLib->CamlinternalMenhirlib renaming).

The issue comes down to the fact that while boot/menhir/parser.ml
is always younger than the parsing/parser.ml produced by promote,
the time difference is very small (parser.ml is copied in boot/
immediately after production), and macos doesn't record creation times
with enough precision to notice it.

This PR removes the temporary parsing/parser.ml created by Menhir
in the promote-menhir rule, so that this file is not seen anymore
by 'make world', and has to be properly reproduced from boot/.

Makefile.menhir

@@ -72,6 +72,7 @@ promote-menhir: parsing/parser.mly
 	    's,^#\(.*\)"[^"]*/menhir/standard.mly",#\1"menhir/standard.mly",g' \
 	    parsing/$$f \
 	    > boot/menhir/$$f; \
+	  rm parsing/$$f; \
 	done
 
 # The import-menhirLib invocation in promote-menhir ensures that each