ot::Spinel::Buffer::OutFrameRemove() method

Removes the current or front output frame from the buffer. If there is an active output from being read (an output frame was prepared earlier with a successful call to `OutFrameBegin()`), this method removes the current active output frame. If there is no current active frame, the front frame in the queue (the next frame which would have been read) will be removed. When a frame is removed all its associated messages will be freed. If the remove operation is successful, this method will invoke the `FrameRemovedCallback` (if not nullptr) before returning the success state.