Simplify wxTextCtrl::GetLastPosition

Last position is just the number of characters in the buffer.

src/msw/textctrl.cpp

@@ -1334,12 +1334,7 @@ wxTextPos wxTextCtrl::GetLastPosition() const
 {
     if ( IsMultiLine() )
     {
-        int numLines = GetNumberOfLines();
-        long posStartLastLine = XYToPosition(0, numLines - 1);
-
-        long lenLastLine = GetLengthOfLineContainingPos(posStartLastLine);
-
-        return posStartLastLine + lenLastLine;
+        return ::GetWindowTextLength(GetHwnd());
     }
 
     return wxTextEntry::GetLastPosition();