hide/show complete row of tabular

I would just do

\documentclass{article}
\begin{document}

%toggles:
\def\rowswitch#1\\{#1\\} %On
%\def\rowswitch#1\\{} %Off


\begin{tabular}{cc}
normal & row\\
\rowswitch toggle & row\\
normal & row
\end{tabular}
\end{document}

I made something enntirely different. See if it suits your needs.

I made a delimited macro that will read everything up to a \\, and a conditional \ifswitch that will or will not print the table row.

I provided two aliases \switchon and \switchoff to make it more intuitive :)

\documentclass{article}

\newif\ifswitch
\let\switchon\switchtrue
\let\switchoff\switchfalse

\def\rowswitch#1\\{%
\ifswitch%
  #1\\
\fi%
}

\begin{document}

\switchon

\begin{tabular}{cc}
normal & row\\
\rowswitch toggle & row\\
normal & row
\end{tabular}

\bigskip

\switchoff

\begin{tabular}{cc}
normal & row\\
\rowswitch toggle & row\\
normal & row
\end{tabular}

\end{document}