revert to upstream pybind repository (moveit#424)

Robert's fork is not required anymore.
However, we can't forward to the current branch head
because it breaks python2 support.

.gitmodules

@@ -1,5 +1,5 @@
 [submodule "core/python/pybind11"]
 	path = core/python/pybind11
-	url = https://github.com/rhaschke/pybind11
+	url = https://github.com/pybind/pybind11
 	branch = smart_holder
 	shallow = true