emacs: Remove conditional from notmuch-fcc-initialization.

The fcc code would only initialize if notmuch-fcc-dirs was set. This was
a problem if you reset the variable, or added the variable later during
initialization. Now we always add the fcc hook, but it doesn't do
anything unless notmuch-fcc-dirs are set.
(cherry picked from commit 80a90787163690d2d87571327ba504a470798c60)