imovie picture-in-picture remove bottom border
You will probably need to crop your source video, which is quite simple to do. Here is a screenshot showing approximately what you're working with:



To crop the clip, first select the clip in the timeline. Then, click the  icon above the video preview, select "Crop to Fill", and resize the rectangle as desired. To prevent it from snapping to the borders, hold the Command key as you drag. Below is an example of what this might look like—though I've cropped it much more than you will, most likely.



Finally, here's what it should look like (I've selected the picture-in-picture tool to demonstrate the bounding box of the clip):