[cleanup] Remove unused GridParallelXEBMetadata class (#6909)

The `GridParallelXEBMetadata` class is not being used anywhere in the
codebase.

Fixes #6601

Author: ayshiff

cirq-core/cirq/experiments/grid_parallel_two_qubit_xeb.py

@@ -49,7 +49,6 @@ def _class_resolver_dictionary() -> Dict[str, ObjectFactory]:
     from cirq.devices.noise_model import _NoNoiseModel
     from cirq.devices import InsertionNoiseModel
     from cirq.experiments import GridInteractionLayer
-    from cirq.experiments.grid_parallel_two_qubit_xeb import GridParallelXEBMetadata
 
     def _boolean_hamiltonian_gate_op(qubit_map, boolean_strs, theta):
         return cirq.BooleanHamiltonianGate(
@@ -142,7 +141,6 @@ def _symmetricalqidpair(qids):
         'GlobalPhaseGate': cirq.GlobalPhaseGate,
         'GridDeviceMetadata': cirq.GridDeviceMetadata,
         'GridInteractionLayer': GridInteractionLayer,
-        'GridParallelXEBMetadata': GridParallelXEBMetadata,
         'GridQid': cirq.GridQid,
         'GridQubit': cirq.GridQubit,
         'HPowGate': cirq.HPowGate,