Fix lint failure in qiskit/extensions/unitary.py (#8366) (#8380)

Recently the pylint CI runs have all started failing because of an
unused OrderedDict import failure in qiskit/extensions/unitary.py. The
OrderedDict usage was removed in #8234 but the import was left in
accidently. This should have been caught by the lint job on that PR but
for some reason it was not caught and has sat there until relatively
recently when pylint started erroring because of the error. Normally for
failures like this they can be attributed to environment differences,
typically a release of pylint or astroid. However, we pin those package
versions because of their tendancy to change behavior, and also a diff
between the installed python packages in CI doesn't show any differences.
Regardless of the underlying cause to unblock CI this commit is necessary
to fix the unused import error.

(cherry picked from commit 007a746)

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

qiskit/extensions/unitary.py

@@ -14,7 +14,6 @@
 Arbitrary unitary circuit instruction.
 """
 
-from collections import OrderedDict
 import numpy
 
 from qiskit.circuit import Gate, ControlledGate