Remove usage of __all__

[mrmundt]

Fixes NA

Summary/Motivation:

We have stopped documenting and using from pyomo.environ import *. We recommend that people explicitly import the modules and elements they desire or use import pyomo.environ as pyo. This removes the usage of __all__ from the entire codebase.

Changes proposed in this PR:

• Remove __all__ from modules
• Removed unused imports

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