remove duplicated drawing of notes

Hi, this PR fixes the problem of drawing notes two times. The problem is usually invisible due to the background color, but if backgroundcolor=none then it becomes clear that something is wrong.

Here is a minimal tex file that shows the problem:

    \documentclass{article}
    \usepackage[backgroundcolor=none]{todonotes}
    \begin{document}
    This text is not sharp. \todo{This text is sharp!}

    Another paragraph:
    \todo[author=Author]{Another todo}
    \end{document}