Remove rqt_gui export

This is not a rqt_gui plugin, so when this package is installed, the following error is printed when running rqt:

RosPluginProvider._parse_plugin_xml() could not parse "[...]/src/graph_rviz_plugin/plugin.xml" in package "graph_rviz_plugin"

This Merge Request fixes that.

This commit should probably also be cherry-picked onto the melodic branch.