Remove graph_reference submodule from .gitmodules.

Author: theengineear

.gitmodules

@@ -1,9 +1,6 @@
 [submodule "submodules/mplexporter"]
 	path = submodules/mplexporter
 	url = git://github.com/mpld3/mplexporter.git
-[submodule "submodules/graph_reference"]
-	path = submodules/graph_reference
-	url = git://github.com/plotly/graph_reference.git
 [submodule "submodules/chunked_requests"]
 	path = submodules/chunked_requests
 	url = git://github.com/chriddyp/chunked_requests.git