Remove the symlinks to vendor directory

## Expected Behavior

The symlinks used in this repo that targets the `vendor` directroy should be removed.

## Actual Behavior

Serving use symlinks to refer YAML files from networking repo:

```
$ find . -type l -printf '%p -> ' -exec readlink {} ';' | grep vendor
./config/core/configmaps/network.yaml -> ../../../vendor/knative.dev/networking/config/config-network.yaml
./config/core/300-imagecache.yaml -> ../../vendor/knative.dev/caching/config/image.yaml
./config/core/300-resources/certificate.yaml -> ../../../vendor/knative.dev/networking/config/certificate.yaml
./config/core/300-resources/domain-claim.yaml -> ../../../vendor/knative.dev/networking/config/domain-claim.yaml
./config/core/300-resources/ingress.yaml -> ../../../vendor/knative.dev/networking/config/ingress.yaml
./config/core/300-resources/serverlessservice.yaml -> ../../../vendor/knative.dev/networking/config/serverlessservice.yaml
```

## Ideas

### Fold the Networking onto Serving

By folding the networking repo onto Serving repo we would remove the need for symlinks across repos.

### Unpack networking YAML files during build

The networking YAML files could be embedded in Go files, and unpacked in Serving in a build process.