mds: remove inappropriate initialization of num_imported

mds: remove inappropriate initialization of num_imported

The variable num_imported is being passed by reference. Additionally,
the decode_import_dir() function is invoked from handle_export_dir(),
where num_imported is initialized and passed by reference.
Therefore, there is no need to initialize it again within
the decode_import_dir() function.

fixes: https://tracker.ceph.com/issues/59107

Signed-off-by: Yongseok Oh <yongseok.oh@linecorp.com>