Remove exponential factor when using custom x-axis labels

Question

I set the xtick-labels to π/2, π and so on. Now because my x-values are small an exponential factor is still drawn under the x-axis. One workaround is to multiply all my values by hand with 1000.

I found an answer how to change the position of this factor here, but how to remove it completely?

The following code demonstrates the problem:

\documentclass[border=3mm]{standalone}
\usepackage{tikz,pgfplots}
\begin{document}
    \begin{tikzpicture}
        \begin{axis}[
            xlabel=$\omega_g$,
            axis lines=middle,
            xmin=0, xmax=0.021,
            xtick = {0, 0.005, 0.01, 0.015, 0.02},
            xticklabels={0, $\frac{\pi}{2}$, $\pi$, 
            $\frac{3 \pi}{2}$, $2\pi$},
        ]
        \addplot []
        table[x=t,y=signal] {
        t       signal
        0       1
        0.01    2
        0.02    3
        };       
      \end{axis}
    \end{tikzpicture}
\end{document}