Remove system checking in UIButton extension

Sources/UIButton+Kingfisher.swift

@@ -24,8 +24,6 @@
 //  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 //  THE SOFTWARE.
 
-#if os(iOS) || os(tvOS)
-
 import UIKit
 
 /**
@@ -300,12 +298,3 @@ extension UIButton {
         kf_backgroundImageTask?.downloadTask?.cancel()
     }
 }
-    
-#elseif os(OSX)
-
-import AppKit
-extension NSButton {
-    // Not Implemented yet.
-}
-    
-#endif