Remove dead code from FlatAffineConstraints

- getDimensionBounds() was added initially for quick experimentation - no
  longer used (getConstantBoundOnDimSize is the more powerful/complete
  replacement).
- FlatAffineConstraints::getConstantLower/UpperBound are incomplete,
  functionality/naming-wise misleading, and not used currently. Removing these;
  complete/fixed version will be added in an upcoming CL.

PiperOrigin-RevId: 225075061