Fix the impl for Displaying zero element in Fp

[winderica]

Description

The current implementation of Display for Fp introduces a bug where formatting Fp::<P, N>::zero() produces an empty string. This is because the leading zeros of a field element will always be removed even when it is zero.

This PR fixes this bug by simply removing the trim_start_matches('0') call, which is in fact unnecessary because self.into_bigint().to_string() (which in turn calls the formatter of num::BigUint) already produces a string without leading zeros.

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