If you are seeking a complete removal of everything outside the Polaroid, the easiest method may be to ignore the drop shadow there, cut out the Polaroid along its edges, then add the shadows back, manually.
It can often be much easier to create subtle shadows than it is to try preserving them when removing surrounding background pixels.
If you merely want to remove the "scum dot", or that slight grey around the Polaroid, you could use a Levels adjustment.
Image > Adjustments > Levels from the menu
When the dialog comes up, there are 3 little eyedropper icons. Click the little eyedropper on the right - this is the "white point" eyedropper.
Now click the area in the image around the Polaroid that it is grey. You should see the histogram in the Levels dialog box change slightly.
What the white point eyedropper does when you click the image is.. it reads the area under the click and sets whatever value is there to be the value used for white. So, basically you are boosting the white level in the image so the grey becomes white. This will adjust the entire image -increasing the white point to be at that slight grey value.
Click okay. And then you'll then need to erase or paint over that dark spot in the lower right corner.