remove documentation for removed make target install-info-debian

* Makefile: also remove the documentation explaining the use of target
  install-info-debian, which was removed previously

TINYCHANGE

Makefile

@@ -527,4 +527,3 @@ targets help:
 	@echo "make install - install Org"
 	@echo "make install-lisp - install Org ELisp files"
 	@echo "make install-info - install Org Info file"
-	@echo "make install-info-debian - install info on old debian systems (newer use ginstall)"