file sender thread was a failed experiment - do_file_senders should only be called once per tox_do iteration

src/windows.h

@@ -67,11 +67,6 @@ struct _Winthread {
     pthread_mutex_t lock;
 };
 
-struct _FSenderThread {
-    pthread_t tid;
-    pthread_mutex_t lock;
-};
-
 typedef struct ToxWindow ToxWindow;
 typedef struct StatusBar StatusBar;
 typedef struct PromptBuf PromptBuf;