helvet package should force professionalfonts

[josephwright]

Came up in https://github.com/josephwright/siunitx/issues/537: the strange font setup doesn't really make sense.

[samcarter]

Combining helvet with the professionalfonts theme will alter the output from sans serif to serif math

\documentclass{beamer}
\usepackage{helvet}
\usepackage{siunitx}

\usefonttheme{professionalfonts}
\begin{document}

\begin{frame}

$\Omega abc$

%\unit{\ohm}

%\DeclareSIUnit{\ohm}{\text{\ensuremath{\Omega}}}

%\unit{\ohm}

\end{frame}

\end{document}