Fix staking contract - unstake function

[portuu3]

Description
Remove first require require(staker.tokensStaked > 0, 'Must be a positive number');, it is unnecessary since tokensStaked>0 will already be validated with this require

require(
            staker.tokensAvailable() >= _tokens,
            'Insufficient amount to unstake'
        );

Motivation
Fix error in the contract

Implementation details
Remove require