You could just mute the undesired track by activating the M (mute) button at the beginning of the audio track. If you want to properly remove the undesired track from the arrangement, so you don't even see it anymore and to make space for the rest of your arrangement, you would have to ungroup audio and video of your imported video file (e.g. by using Ctrl+M or by using the broken chain icon in the tool bar). Then you can just select and delete the audio track from it.