CTRL+SHIFT+A is already taken - per default, it’s assigned to “Everything in column, above cursor” and works in conjunction with CTRL+SHIFT+Z (everything below).
I would personally prefer if we had a keyboard shortcut which could select all (CTRL+A), and then, when triggered again, deselect everything.
So, it would have a bit of intelligence - only when the selection does in fact span the entire pattern will it deselect.
Worst case? Hit CTRL+A two times.
Read your other post on keyboard shortcuts too, btw. 