Removes nsIWindowsShellService.idl since there are no longer custom methods and it can just use nsIShellService.idl
Removes a couple of includes that are no longer used and should have been removed after the code to delete Firefox prefetch files was removed
In addition to fixing the bug, this patch takes the opportunity to remove a
bunch of registry handling code specific to XP and Vista, leaving only the one
officially supported default detection method for all versions above Vista.
MozReview-Commit-ID: 1I77ECJaOFk
