documentation: api_reference.md: removed Swift reference
See the following API reference docs for the Mobile SDK
* [Android (Java) API](http://cartodb.github.io/mobile-android-samples/)
-* [iOS (Objective-C, Swift) API](http://cartodb.github.io/mobile-ios-samples/)
+* [iOS (Objective-C) API](http://cartodb.github.io/mobile-ios-samples/)
