Revert storing $ios_ifa in automatic properties for now

Author: zihejia

Mixpanel/AutomaticProperties.swift

@@ -63,9 +63,6 @@ class AutomaticProperties {
         p["$ios_device_model"]  = AutomaticProperties.deviceModel()
         #if !os(OSX)
         p["$ios_version"]       = UIDevice.current.systemVersion
-        if let ifa = Mixpanel.mainInstance().IFA() {
-            p["$ios_ifa"] = ifa;
-        }
         #else
         p["$ios_version"]       = ProcessInfo.processInfo.operatingSystemVersionString
         #endif // os(OSX)