Fix wrong assert in LookupTable linear mapping

Removing wrong assert() condition following discussion on MR !3408
Only the truncation of the index should be in a valid range. The
original double value for dIndex might be above the range due to lack
of precision.

Common/Core/vtkLookupTable.cxx

@@ -556,8 +556,7 @@ inline vtkIdType vtkLinearIndexLookupMain(double v, const TableParameters & p)
     // dIndex may map above the highest value in the lut (at index p.NumColors-1)
     // in the linear mapping above. This is why we keep an extra copy of the last
     // lut value, to avoid extra work in this very hot function.
-    // It should never be more than 1 off, assert to be sure.
-    assert(dIndex > -1.0 && dIndex <= (double)p.NumColors && std::isfinite(dIndex));
+    // It should never be more than 1 off after the following truncation.
     index = static_cast<vtkIdType>(dIndex);
   }