removed older version of matrix product operator

src/__init__.py

@@ -46,7 +46,6 @@ from .operators.simple_linear_operators import (
     VdotOperator, ConjugationOperator, Realizer, FieldAdapter, ducktape,
     GeometryRemover, NullOperator, PartialExtractor, Imaginizer)
 from .operators.matrix_product_operator import MatrixProductOperator
-from .operators.old_matrix_product_operator import OldMatrixProductOperator
 from .operators.value_inserter import ValueInserter
 from .operators.energy_operators import (
     EnergyOperator, GaussianEnergy, PoissonianEnergy, InverseGammaEnergy,