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.cpp

@@ -37,12 +37,6 @@ void SetfLargeWorkInvalidChainFound(bool flag)
     fLargeWorkInvalidChainFound = flag;
 }
 
-bool GetfLargeWorkInvalidChainFound()
-{
-    LOCK(cs_warnings);
-    return fLargeWorkInvalidChainFound;
-}
-
 std::string GetWarnings(const std::string& strFor)
 {
     std::string strStatusBar;