remove unused codes for coverage fix (#227)

Author: jaeopt

OptimizelySDK/Protocols/BackgroundingCallbacks.swift

@@ -24,22 +24,14 @@ import UIKit
 
 extension BackgroundingCallbacks {
     func subscribe() {
-        #if swift(>=4.2)
+        // swift4.2+
         NotificationCenter.default.addObserver(self, selector: #selector(applicationDidEnterBackground), name: UIApplication.didEnterBackgroundNotification, object: nil)
         NotificationCenter.default.addObserver(self, selector: #selector(applicationDidBecomeActive), name: UIApplication.didBecomeActiveNotification, object: nil)
-        #else
-        NotificationCenter.default.addObserver(self, selector: #selector(applicationDidEnterBackground), name: .UIApplicationDidEnterBackground, object: nil)
-        NotificationCenter.default.addObserver(self, selector: #selector(applicationDidBecomeActive), name: .UIApplicationDidBecomeActive, object: nil)
-        #endif
     }
 
     func unsubscribe()  {
-        #if swift(>=4.2)
+        // swift4.2+
         NotificationCenter.default.removeObserver(self, name: UIApplication.didEnterBackgroundNotification, object: nil)
         NotificationCenter.default.removeObserver(self, name: UIApplication.didBecomeActiveNotification, object: nil)
-        #else
-        NotificationCenter.default.removeObserver(self, name: .UIApplicationDidEnterBackground, object: nil)
-        NotificationCenter.default.removeObserver(self, name: .UIApplicationDidBecomeActive, object: nil)
-        #endif
     }
 }