emacs: turn off auto-fill-mode for text-mode

Previously, I turned on auto-fill-mode in text-mode to enable auto-fill
in git commit messages. Turned out that this is not the best decision
ever, since auto-fill is propagated to markdown mode. I found out from
http://www.cmake.org/Wiki/ITK/Git/TipsAndTricks that we can turn on
auto-fill-mode specifically for COMMIT_EDITMSG. So now I've disabled
auto-fill-mode for text-mode. Markdown editing is pleasant again.