[fluid remove]: remove paddle.fluid.layers.target_assign, paddle.fluid.layers.rpn_target_assign, paddle.fluid.layers.retinanet_target_assign and paddle.fluid.layers.ssd_loss (#48669)

* remove paddle.fluid.layers.nn.temporal_shift

* code check

* rm unittest

* remove ssd_loss, target_assigns

* remove target_assign, retinanet_target_assign, rpn_target_assign and ssd_loss