electra SequenceClassification layer change
Author: kamalkraj
Pull request: yes

# What does this PR do?

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Fixes # (issue)
from `ElectraClassificationHead` removed extra projection layer to make the fine-tuning models  architure even with the original implementation. 
As the ELECTRA pre-train doesn't have a sentence contrastive task there is no pooling layer for ELECTRA.
Code in the original codebase -
1. No additional projection, just returning the CLS token representation.
https://github.com/google-research/electra/blob/8a46635f32083ada044d7e9ad09604742600ee7b/model/modeling.py#L266
2. pooling_output -> dropout -> classification_dense 
https://github.com/google-research/electra/blob/8a46635f32083ada044d7e9ad09604742600ee7b/finetune/classification/classification_tasks.py#L218

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