remove/cli: drop unused validate commands (#1310)

The legacy `src validate` entrypoint is unused, so keeping it around adds dead code and extra maintenance surface in the CLI. This removes the top-level `src validate` command and its `install` and `kube` subcommands, which were still wired through the older command registration path. For more context, see https://sourcegraph.slack.com/archives/C022SPMNR0W/p1777905299691889

## Test Plan

- go test ./cmd/src/...

Author: burmudar