chaincfg: Deprecate subsidy split params.
chaincfg: Deprecate subsidy split params.

This deprecates the per-network subsidy split parameters for the
following reasons.

- They are the same on all networks so there really is no good reason to
  parameterize them as such
- The subsidy calculation code can be simplified and made slightly more
  efficient without the unnecessary parameterization

decred/dcrd,decred/dcrd