Redundant forward declaration

(cherry picked from commit 97b93d813cbbf1a90aa7e9c2c82216b810ab7b41)

Source commit: 8650317266c6d5e7f771c588439f2a02ecde7817

source/filezilla/FtpControlSocket.h

@@ -11,7 +11,6 @@ class CTransferSocket;
 class CMainThread;
 //---------------------------------------------------------------------------
 class CAsyncProxySocketLayer;
-class CMainThread;
 //---------------------------------------------------------------------------
 #define CSMODE_NONE             0x0000
 #define CSMODE_CONNECT          0x0001