Removed unused `encoder_hidden_states` and `encoder_attention_mask` 
# What does this PR do?

This PR removes unused `encoder_hidden_states` and `encoder_attention_mask` from MobileBert forward methods. These are use for decoder models, but MobileBert does not include a cross-attention mechanism.

Fixes https://github.com/huggingface/transformers/issues/8969


## Who can review?
 albert, bert, XLM: @LysandreJik

@LysandreJik I had to remove some tests that were testing the decoder mode for MobileBert. 

One test still fails (flax), the error seems unrelated to this PR unless I am missing something?