Revert "Add support for empty and string maildir shortcuts"

This reverts commit be695579c6.

The change didn't look very nice (keynames are not aligned etc.), and in
hindsight we should probably implement this slightly differently.

mu4e/mu4e-main.el

@@ -210,7 +210,7 @@ clicked."
            with longest = (mu4e--longest-of-maildirs-and-bookmarks)
            with queries = (plist-get mu4e--server-props :queries)
            for m in mds
-           for key = (plist-get m :key)
+           for key = (string (plist-get m :key))
            for name = (plist-get m :name)
            for query = (plist-get m :query)
            for qcounts = (and (stringp query)