Remove david-dm from README

[renbaoshuo]

The david-dm.org website is down for 3 months, and the project alanshaw/david is not actively maintained anymore. david-dm.org should be considered dead, thus badges should be removed from README.

Checklist

• [x] npm run test passes
• [ ] tests and/or benchmarks are included
• [ ] cases or donate is changed or added
• [x] documentation is changed or added

[tencent-adm]

All committers have signed the CLA.