how to suppress page number for subfigures in LOF?

Here's one possibility which enables you to use all of tocloft features for subfigure entries when using the subcaption package; in particular, you can suppress the page numbers and the leaders (the dots from the entry to the page number):

\documentclass{article}
\usepackage[list=true]{subcaption}
\usepackage{tocloft}

\setcounter{lofdepth}{2}
\cftpagenumbersoff{subfigure}
\makeatletter
\newcommand\cftsubtabdotsep{\cftdotsep}
\newcommand\cftsubfigfont{\normalfont}
\newcommand\cftsubfigpresnum{}
\newcommand\cftsubfigaftersnum{}
\newcommand\cftsubfigaftersnumb{}
\newcommand\cftsubfigleader{\hfill}
\newcommand\cftsubfigpagefont{\normalfont}
\newcommand\cftsubfigafterpnum{}
\providecommand{\toclevel@subfigure}{1}
\renewcommand*\l@subfigure[2]{%
  \ifnum \c@lofdepth > \toclevel@subfigure
    \vskip \cftbeforesubfigskip
    {\leftskip \cftsubfigindent\relax
     \rightskip \@tocrmarg
     \parfillskip -\rightskip
     \parindent \cftsubfigindent\relax\@afterindenttrue
     \interlinepenalty\@M
     \leavevmode
     \@tempdima \cftsubfignumwidth\relax
     \let\@cftbsnum \cftsubfigpresnum
     \let\@cftasnum \cftsubfigaftersnum
     \let\@cftasnumb \cftsubfigaftersnumb
     \advance\leftskip \@tempdima \null\nobreak\hskip -\leftskip
     {\cftsubfigfont #1}\nobreak
     \cftsubfigfillnum{#2}}%
  \fi
  }%
\newlength\cftbeforesubfigskip
  \setlength\cftbeforesubfigskip{\z@ \@plus.2\p@}
\newlength\cftsubfigindent
  \setlength\cftsubfigindent{3.8em}
\newlength\cftsubfignumwidth
  \setlength\cftsubfignumwidth{2.5em}
\renewcommand{\cftsubfigfillnum}[1]{%
  {\cftsubfigleader\hfill}\par
}
\makeatother

\begin{document}

\listoffigures

\begin{figure}
\begin{subfigure}{.5\linewidth}
\centering
A
\caption{test subfigure A}
\label{sfig:testa}
\end{subfigure}%
\begin{subfigure}{.5\linewidth}
\centering
B
\caption{test subfigure B}
\label{sfig:testb}
\end{subfigure}
\caption{Test figure}
\label{fig:testa}
\end{figure}

\begin{figure}
\begin{subfigure}{.5\linewidth}
\centering
C
\caption{test subfigure C}
\label{sfig:testc}
\end{subfigure}%
\begin{subfigure}{.5\linewidth}
\centering
D
\caption{test subfigure D}
\label{sfig:testd}
\end{subfigure}
\caption{Test figure}
\label{fig:testb}
\end{figure}

\end{document}