1. Add `centipede` to `DEFAULT_ENGINES`.
2. Remove the `fuzzing_engines` attribute from `project.yaml`s if
redundant (i.e., all default engines are included in that YAML file).
3. Temporarily disable `centipede` on `c`/`c++` projects that previously
use all `DEFAULT_ENGINES` (200 projects did not specify
`fuzzing_engines` in `project.yaml`) so that we can gradually enable
centipede later them later.
Dongge Liu