Tiny toplevel www/Makefile nit.

The <bsd.obj.mk> makefile include already pulls in <bsd.subdir.mk>,
so including the later again causes spurious warnings about targets
being redefined, like:

"/usr/share/mk/bsd.subdir.mk", line 57: warning: duplicate script for target "_SUBDIR" ignored
"/usr/share/mk/bsd.subdir.mk", line 66: warning: duplicate script for target "en" ignored
"/usr/share/mk/bsd.subdir.mk", line 84: warning: duplicate script for target "files" ignored
"/usr/share/mk/bsd.subdir.mk", line 84: warning: duplicate script for target "includes" ignored

Removing the extra <bsd.subdir.mk> include fixes this.