Merge ocamldoc/Makefile.best_ocamldoc into Makefile.best_binaries
Makefile.best_ocamldoc was included in two files:
1. In ocamldoc/Makefile, which also includes
$(ROOTDIR)/Makefile.best_binaries
So that the definitions which were in Makefile.best_ocamldoc remain available.
2. In api_docgen/ocamldoc/Makefile which also includes
Makefile.best_binaries via api_docgen/Makefile.common