Remove unreachable code

[jj1980a]

Related audit issue: https://github.com/spearbit-audits/review-fastlane/issues/117

Removing unreachable code in _assign(). amount can never be 0.

[ephess]

OK, so I'm missing a bit of AtlETH knowledge here, but from what I can see:

• When we want to unbond AtlETH, it goes through an escrow period.
• During this escrow period, any time we access the account and lastAccessedBlock is updated, the escrow period resets.

If that's the case, then this change essentially means that the intent of the code from before the change, was that any time a solvers bid was considered as part of an ex post bids transaction, we would reset the escrow period. But this was never possible due to where this function is called from, so this code never executes, meaning we're not losing any behaviour from removing the code.

Looks good to me.