Remove spurious SXGate._ARRAY (Qiskit#10876)

* Remove spurious `SXGate._ARRAY`

This was left in only as a mistake during the writing of Qiskit#10296;
originally the `with_gate_array` decorator didn't exist and all classes
had manual `_ARRAY` specifiers, but `SXGate`'s got left in accidentally
when all the rest were removed in favour of the decorator.

* Remove unused import

qiskit/circuit/library/standard_gates/sx.py

@@ -14,7 +14,6 @@
 
 from math import pi
 from typing import Optional, Union
-import numpy
 from qiskit.circuit.controlledgate import ControlledGate
 from qiskit.circuit.singleton_gate import SingletonGate
 from qiskit.circuit.quantumregister import QuantumRegister
@@ -146,10 +145,6 @@ class SXdgGate(SingletonGate):
                     = e^{-i pi/4} \sqrt{X}^{\dagger}
 
     """
-    _ARRAY = numpy.array(
-        [[0.5 - 0.5j, 0.5 + 0.5j], [0.5 + 0.5j, 0.5 - 0.5j]], dtype=numpy.complex128
-    )
-    _ARRAY.setflags(write=False)
 
     def __init__(self, label: Optional[str] = None, duration=None, unit=None, _condition=None):
         """Create new SXdg gate."""