Remove indentation at the beginning of subsections (using amsbook)

Is there a way to get rid of the indentation at the beginning of subsections when using the documentclass amsbook?

See the attached image for an example (where the problematic indentation is marked blue):

The example was generated using the following code:

documentclass[a4paper]{amsbook}
usepackage{lipsum}

begin{document}

chapter{This is a chapter}
section{This is a section}
subsection{This is a subsection}
lipsum[1-2]

end{document}

documentclass[a4paper]{amsbook}
usepackage{lipsum}

makeatletter
defsubsection{@startsection{subsection}{2}%
    z@{[email protected]}{-.5em}%
    {normalfontbfseries}}
makeatother

begin{document}

chapter{This is a chapter}
section{This is a section}
subsection{This is a subsection}
lipsum[1-2]

end{document}