Cleanup GetBlockValue and GetMasternodePayment

[poiuty]

Now we have ASIC and dynamic reward will soon stop working. So, let's simplify code and cleanup it.
Also I think we are no longer needed if else and separation of main or test net.

[UdjinM6]

nACK

1. I have no crystal ball to say that diff will only rise.
2. even if we accept this theory and decide to change block reward to some constant value one day making changes this way is absolutely unacceptable because this will break validation for historical blocks.

[schinzelh]

NACK as well:

• this breaks validation of historic blocks and
• is actually a hard fork

So I neither agree on the concept nor on its implementation.