By default in PGFplots, when axes are only shown in the bottom left corner, arrows appear at the end of the ends of axes lines. How can I change the defaults so these arrows do not appear?
Edit: Example code that produces plots with arrows at the tips of the axes lines
\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[axis x line=middle,axis y line=middle]
\addplot {x^3};
\end{axis}
\end{tikzpicture}
\end{document}

