Remove unnecessary pre-19 code branches.

Now that our minSdk is 19, we can remove a lot of old code paths that
only ran pre-19.

src/org/thoughtcrime/securesms/notifications/SingleRecipientNotificationBuilder.java

@@ -220,7 +220,7 @@ public class SingleRecipientNotificationBuilder extends AbstractNotificationBuil
   }
 
   private boolean hasBigPictureSlide(@Nullable SlideDeck slideDeck) {
-    if (slideDeck == null || Build.VERSION.SDK_INT < 16) {
+    if (slideDeck == null) {
       return false;
     }