remove some unused functions

Identified with `cppcheck --enable=unusedFunction .`.

 - GetSendBufferSize()'s last use removed in
   991955ee81
 - SetPort()'s last use removed in
   7e195e8459
 - GetfLargeWorkInvalidChainFound() was introduced in
   e3ba0ef956 and never used

src/warnings.h

@@ -13,7 +13,6 @@ void SetMiscWarning(const std::string& strWarning);
 void SetfLargeWorkForkFound(bool flag);
 bool GetfLargeWorkForkFound();
 void SetfLargeWorkInvalidChainFound(bool flag);
-bool GetfLargeWorkInvalidChainFound();
 /** Format a string that describes several potential problems detected by the core.
  * strFor can have three values:
  * - "rpc": get critical warnings, which should put the client in safe mode if non-empty