notmuch new: Remove an unnecessary stat of every regular file in the mail store.

We were previousl using the stat for two reasons. One was to obtain
the mtime of the file. This usage was removed in the previous commit,
(since the mtime is unreliable in the case of a file being moved into
the mail store).

The second reason was to identify regular and directory file
types. But this information is already available in the result we get
from scandir.

What's left is simply a stat for each directory in the mailstore,
(which we are still using to compare filesystem mtime with the mtime
stored in the database).