NO-JIRA: Clean up unused Dockerfile and Dockerfile.rhel7

Follow up #1236 (comment)

The benifit is we do not need to maintain them such
as #1236

* The `Dockerfile` was not used in [CI configuration](https://github.com/openshift/release/tree/master/ci-operator/config/openshift/cluster-version-operator).
  and not in [ocp-build](https://github.com/openshift-eng/ocp-build-data/blob/ee5e00d600bd2d15cf36d9c395c0c333106f3489/images/cluster-version-operator.yml#L3) either. Both of them
  use `Dockerfile.rhel`. Petr also [confirms](10deb6f)
  this finding and suggests that it is there is only
  for external contributors. However, it stops working
  anyway.

* By replacing the private images in `Dockerfile.rhel`
  with relatively well-known public ones, the build
  can be recovered. So the bar is relatively low for
  the externals.

```console
$ diff --git a/Dockerfile.rhel b/Dockerfile.rhel
  index fede93e..cf549c0a 100644
  --- a/Dockerfile.rhel
  +++ b/Dockerfile.rhel
  @@ -1,11 +1,10 @@
  -FROM registry.ci.openshift.org/ocp/builder:rhel-9-golang-1.24-openshift-4.21 AS builder
  -WORKDIR /go/src/github.com/openshift/cluster-version-operator
  +FROM docker.io/golang:1.24 AS builder
   COPY . .
   RUN hack/build-go.sh; \
       mkdir -p /tmp/build; \
       cp _output/linux/$(go env GOARCH)/cluster-version-operator /tmp/build/cluster-version-operator

  -FROM registry.ci.openshift.org/ocp/4.21:base-rhel9
  +FROM registry.access.redhat.com/ubi9/ubi-minimal:latest
   COPY --from=builder /tmp/build/cluster-version-operator /usr/bin/
   COPY install /manifests
   COPY vendor/github.com/openshift/api/config/v1/zz_generated.crd-manifests/0000_00_cluster-version-operator_* /manifests/
```

* `Dockerfile.rhel7` was kept only for [backwards-compatibility](32204c0).
  It has been long enough (over 5 years already) for
  the consumers to adapt.

Author: hongkailiu