Monobook: Remove OperaNFixes.css files

We were providing fixes for Opera 6, 7 and 9, even though it's 2013,
Opera is at version 12, and even as a grade B browser we're only
supporting Opera 10.

Given the amount of various changes to the codebase over the years,
I'd be surprised if any of those were still doing anything for the
compatibility.

Change-Id: I9925cc98d11c4e2eecedd305a724f0763a62a782
Merge "Monobook: Remove OperaNFixes.css files"