Summary/Motivation:

This is a non-functional change which removes some references to contrib.simplemodel and pyomocontrib_simplemodel that were missed when that module was deprecated and removed years ago. This also removes an old documentation index file that should have been deleted when we did the documentation reorganization.

Changes proposed in this PR:

• Remove references to simplemodel in pyomo.contrib
• Remove old pyomo.contrib documentation index file

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