The value of $$\left(\sin 30^{\circ} \cos 60^{\circ} - \cos 30^{\circ} \sin 60^{\circ}\right)$$ is equal to:

A

$$-\cos 30^{\circ}$$

B

$$-\sin 30^{\circ}$$

C

$$\cos 30^{\circ}$$

D

$$\sin 30^{\circ}$$