migration: multifd_save_cleanup() can't fail, simplify

multifd_save_cleanup() takes an Error ** argument and returns an
error code even though it can't actually fail.  Its callers
dutifully check for failure.  Remove the useless argument and return
value, and simplify the callers.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Fei Li <fli@suse.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20190113140849.38339-4-lifei1214@126.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
diff --git a/migration/migration.c b/migration/migration.c
index 24cb4b9..5d322eb 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1386,7 +1386,6 @@
     qemu_savevm_state_cleanup();
 
     if (s->to_dst_file) {
-        Error *local_err = NULL;
         QEMUFile *tmp;
 
         trace_migrate_fd_cleanup();
@@ -1397,9 +1396,7 @@
         }
         qemu_mutex_lock_iothread();
 
-        if (multifd_save_cleanup(&local_err) != 0) {
-            error_report_err(local_err);
-        }
+        multifd_save_cleanup();
         qemu_mutex_lock(&s->qemu_file_lock);
         tmp = s->to_dst_file;
         s->to_dst_file = NULL;