Remove prepareMsg (and OUT_PREPARING message state) (#3468)

This PR removes prepareMsg, which put messages into the OUT_PREPARING state and was used while videos were recoded. It also removes the now-unused method isIncreation().
- It was buggy because when you forwarded a message while it was InPreparation, or when the app was killed while a message is InPreparation, the message would stay InPreparation forever.
- Android is the only UI using this InPreparation (according to @r10s, I didn't check this myself), so we can simplify some things in core, which will also make it easier to deduplicate blob files.

src/main/java/com/b44t/messenger/DcContext.java

@@ -194,7 +194,6 @@ public class DcContext {
     public native void         deleteMsgs           (int msg_ids[]);
     public native void         forwardMsgs          (int msg_ids[], int chat_id);
     public native boolean      resendMsgs           (int msg_ids[]);
-    public native int          prepareMsg           (int chat_id, DcMsg msg);
     public native int          sendMsg              (int chat_id, DcMsg msg);
     public native int          sendTextMsg          (int chat_id, String text);
     public native int          sendVideochatInvitation(int chat_id);

src/main/java/com/b44t/messenger/DcMsg.java

@@ -155,7 +155,6 @@ public class DcMsg {
     public native String  getSetupCodeBegin  ();
     public native String  getVideochatUrl    ();
     public native int     getVideochatType   ();
-    public native boolean isIncreation       ();
     public native void    setText            (String text);
     public native void    setFile            (String file, String filemime);
     public native void    setDimension       (int width, int height);