Chainparams: Remove CElementsParams
Chainparams: Remove CElementsParams

was:

- MOVEONLY: Move CMainParams after CCustomParams
- Chainparams: Now CMainParams depends on CRegTestParams instead of CElementsParams
- Chainparams: Remove unused CElementsParams

tdudz/elements,tdudz/elements,tdudz/elements,tdudz/elements,tdudz/elements,tdudz/elements