Delete .ko.yaml

The default base image for `ko` since v0.12.0 is `cgr.dev/chainguard/static`
This is a very lightweight base image with just the bare minimum required to run static binaries.
It also runs as nonroot by default so matches the previous image.

Comparison of base image sizes (output from `docker images`):

```
cgr.dev/chainguard/static        latest      90320c6851cc  6 weeks ago    3.06 MB
gcr.io/distroless/base-debian11  nonroot     053d24ed112e  4 seconds ago  19.3 MB
```

Comparison of built Dashboard image (using installer script with default settings):

```
root@tekton-dashboard-control-plane:/# crictl images | grep dashboard
kind.local/dashboard-9623576a202fe86c8b7d1bc489905f86                  latest
921213af83dc0       23.2MB
kind.local/dashboard-9623576a202fe86c8b7d1bc489905f86
94370ac46698f930b2b8366a1f6d2a9ca3f24080c20a440106bc1599d8decc76   921213af83dc0       23.2MB
kind.local/dashboard-9623576a202fe86c8b7d1bc489905f86
ecc31e47e5b7e50c1b9c92805d3c4b69d6425f9757677fdaa800b008d198487c   e5e5e2f4101d1       31.3MB
```

This gives an over 25% reduction in the final image size.