[build] Target Mainnet for Develop Image

I'm setting up a node that will run on the latest image from develop and be reset daily from the latest statesync snapshot, alerting if it hits a consensus failure. This will change the image registry.gitlab.com/thorchain/thornode:develop to be built for mainnet instead of mocknet. Additionally this removes the build tag mappings for the master and testnet-multichain branches since they are no longer used.