migration/multifd: Remove error_setg() in migration_ioc_process_incoming()

migration/multifd: Remove error_setg() in migration_ioc_process_incoming()

If multifd_load_setup() fails in migration_ioc_process_incoming(),
error_setg() is called with errp. This will lead to an assert because in
that case errp already contains an error.

Fix it by removing the redundant error_setg().

Fixes: 6720c2b32725 ("migration: check magic value for deciding the mapping of channels")
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20231231093016.14204-9-avihaih@nvidia.com
Signed-off-by: Peter Xu <peterx@redhat.com>