Able to update payment amount for Workperiodpayments even when challenge is completed

[sandhiyakavi]

Description:

Able to update payment amount for Workperiodpayments even when challenge is completed.

The amount is not changed in Challenge as it is in 'Completed' status

[SathyaJayabal]

@maxceem , once payment status is complete, there should not be any updates to the record.

[maxceem]

We no longer can update amount as per #288.

[sandhiyakavi]

Verified on dev Env. We are no longer able to update amount

[sandhiyakavi]

Verified on Prod Env with @nkumar-topcoder .