Variable recently_missed_count is no longer in use

**User Story**

The variable `recently_missed_count` in dynamic global properties is no longer in use since https://github.com/bitshares/bitshares-core/commit/3ded0d4e773594361a7fe2db40cef0518c257f78.

It was useful for playing with checkpoints, see https://github.com/bitshares/bitshares-core/commit/73cc012db90fce64671a6669af3cd7671a49be28. Now the trick to restart block production with a dummy checkpoint (a known block in the past with its correct ID) no longer works when undo_db and fork_db are growing. Another trick still works -- setting an unknown block in the future as a checkpoint -- but it is too dangerous in production since validation will be skipped.

Perhaps implement another trick, or remove related code.

We did not use checkpoints in the BitShares Mainnet so far.

**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [ ] API (the application programming interface)
- [ ] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation