Remove dead code branch from Optimize1qDecomposition Python code (#…

…13188)

Since 8e5fab6 (gh-12959), this branch has been dead, and the Rust
method alluded to in it (`compute_error_one_qubit_sequence`) was
removed.  This is occasionally - but for some reason not always -
detected by `pylint`, and it's a true error so should be removed.

In fact, almost all of the Python code in `Optimize1qDecomposition` is
dead from the perspective of that transpiler pass.  However,
`Optimize1qCommutation` has no concept of safe API boundaries, and is in
fact still using all of these methods (and the default
`UnitarySynthesis` plugin does a little too).  A follow-up commit can
reorganise the code to fix this, but this commit is intended to be
mergeable faster, to unblock a couple of PRs that are triggering the
pylint failure.

qiskit/transpiler/passes/optimization/optimize_1q_decomposition.py

@@ -222,18 +222,12 @@ def run(self, dag):
     def _error(self, circuit, qubit):
         """
         Calculate a rough error for a `circuit` that runs on a specific
-        `qubit` of `target` (`circuit` can either be an OneQubitGateSequence
-        from Rust or a list of DAGOPNodes).
+        `qubit` of `target` (`circuit` is a list of DAGOPNodes).
 
         Use basis errors from target if available, otherwise use length
         of circuit as a weak proxy for error.
         """
-        if isinstance(circuit, euler_one_qubit_decomposer.OneQubitGateSequence):
-            return euler_one_qubit_decomposer.compute_error_one_qubit_sequence(
-                circuit, qubit, self.error_map
-            )
-        else:
-            return euler_one_qubit_decomposer.compute_error_list(circuit, qubit, self.error_map)
+        return euler_one_qubit_decomposer.compute_error_list(circuit, qubit, self.error_map)
 
 
 def _possible_decomposers(basis_set):