fixed warning with asset catalog when building with Xcode 5 / iOS 7 SDK

Fixed a build warning under Xcode 5 / iOS 7 SDK when compiling the 1Password.xcassets asset catalog.

The Xcode 5 warnings were:
The image set "extension-ipad" has an unassigned image.
The image set "extension-iphone" has an unassigned image.

I simply deleted the extra empty image.