Step 1 --> Straighten the page by using the 'Measure Tool.'
This is how the page should look like right now.
I made sure the background color was black, so you're able to see it better.
Step 2 --> Try to redraw as much as possible with the 'Healing Brush Tool' first. Like the examples below.
This example turned out not to be a good one, because I'm actually able to redraw everything without cropping the page.
So... I just skipped doing the top part.
Step 3 --> Click with your mouse on the 'Crop Tool'.
Step 4 --> Click with your mouse on the top right end of the page and drag it all the way to the bottom
left end of the page.
If done right, it should look like this.
At the top of your screen, you should see this toolbar.
These settings above mean that what I'm about to crop away will turn black for 75%.
Step 5 --> Click with your mouse on the top 'dotted' line and drag it like the example below.
If you let go of the mouse your page will look like this.
When you've decided how much you want to crop the page hit on the 'Enter'-key. This is to apply the transformation.
Final outcome should be like this.