Can't just not look at it? What is the real harm to your work other than you do not care?
You can collapse the left-side pane by clicking the small arrow between this pane and the document window. The arrow will be still there, so that you can bring it back if you want. I doubt there is a preference to suppress it but you can look through the preferences, maybe there is one, I just do not know and in my work the current behavior suites me just fine.
One of the things I have to do is approve PDF proofs send by vendors. The most foolproof way to do this is to open both PDFs (ours and theirs) at the same zoom and then CMD-~ to toggle between them. Any differences will be immediately obvious.
Our PDFs show this warning bar, theirs do not. So every time I go to do this I have to turn off the blue bar, and then the sidebar. Not a big deal for one or a few, but when you're doing this many tens of times a day it gets very tiresome.
There should be a way to turn this warning off, for those who do not need it or do not care about it. If there isn't, then I'd submit it as a feature request.