Revise IO and plugin subsystem text

Replace the IO subsystem's dependency description with
placeholder notes marking its responsibilities and
relationship with FORM as currently unclear.

Remove Data Modeling from the Plugin Management subsystem
dependency description.

Author: marcpaterno

doc/supporting_design.rst

@@ -37,11 +37,10 @@ The 14 core subsystems and their dependency arrows show the following structure:
 
 
 - **IO** provides the mechanisms by which data products and relevant metadata are read and written to persistent storage.
+  It is unclear exactly what the responsibilities of the IO subsystem are.
+  It is unclear what the relationship betwen the IO subsystem and FORM is.
 
-  depends on *Data Modeling*, meaning the IO subsystem uses the data model to
-  know how to read and write products.
-
-- **Plugin Management** and **Data Modeling** both depend on *Registration* and
+- **Plugin Management** depend on *Registration* and
   *Monitoring*, contributing discovered plugins and data-model events to those subsystems
   respectively.