Remove CBaseChainParams::NetworkID()

Continues #4802 and #4803.
Completes the set of PRs #3823 #3824 #4333 #4801 #4802 #4803 
(huh I thought I commented this before but I lost my comment)
Don't move this GUI decision to the core code. The GUI needs to be able to make the decision for network-based theming based on some identifier of the chain. If you insist on removing the enumeration you could match on `NetworkIDString()`.