fix: delete unused layer `z_proj`

[gaozhihan]

1. delete unused layer z_proj.
2. deprecate old pretrained weights to https://deep-earth.s3.amazonaws.com/experiments/earthformer/pretrained_checkpoints/_deprecated_z_proj_earthformer_earthnet2021.pt
3. update new pretrained weights using the same url as the previous: https://deep-earth.s3.amazonaws.com/experiments/earthformer/pretrained_checkpoints/earthformer_earthnet2021.pt

[gaozhihan]

The unittest has passed, which means the new nn.Module load the new state_dict correctly. https://github.com/amazon-science/earth-forecasting-transformer/blob/a01ea56de4baddc5b381757e5d789f7f1efdcffe/tests/unittests/test_pretrained_checkpoints.py#L171-L173