Remove david-dm from README

[renbaoshuo]

Description

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.

Related Issue

Screenshots or GIFs (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[ukutaht]

Thanks!