*: cleanup main_test.go

The etcd-io#15544 has removed the `build_cov` build. And after go1.20, we use
`-cover` buildflag to enable coverage exporter. We don't need to
maintain main_test.go anymore.

```bash
➜  pwd
/home/fuwei/go/src/go.etcd.io/etcd/etcdctl

➜  go build -o /tmp/etcdctl -cover ./

➜  mkdir /tmp/etcdctl-covdata

➜  GOCOVERDIR=/tmp/etcdctl-covdata /tmp/etcdctl get /health

➜  go tool covdata percent -i=/tmp/etcdctl-covdata
        go.etcd.io/etcd/etcdctl/v3      coverage: 66.7% of statements
        go.etcd.io/etcd/etcdctl/v3/ctlv3        coverage: 83.3% of statements
        go.etcd.io/etcd/etcdctl/v3/ctlv3/command        coverage: 15.4% of statements
```

REF: https://go.dev/testing/coverage/

Signed-off-by: Wei Fu <fuweid89@gmail.com>