conditional fix for specific linux wallet crash

[zettel-kasten]

concern qt wallet for linux.
When clicking on mining tab start/stop mining or changing mining threads the wallet will crash.

Removing this code in miningpage.cpp helps:

if (fGenerate && !hasMiningprivkey && !unlockContext.get())
    {
        this->unlockContext.reset(new WalletModel::UnlockContext(model->requestUnlock()));
        if (!unlockContext->isValid())
        {
            unlockContext.reset(NULL);
            return;
        }
    }

Not sure why exactly this happens, but this provides a temporary solution until we know how to handle this.