test: remove unnecessary integration tests layers

Drop `hack/test-prerequisites.go` because it was only used to install CRDs that
will be present on all OpenShift clusters (I dont think testing against plain
k8s clusters is something that we ever needed). With new CRDs such as
`ClusterVersionOperator` this gets stale, and the script did not take feature
sets into account either.

Drop the `hack/test-integration.sh` because its only purpose was to run the
prerequisite installation dropped above before `go test`. `Makefile` can easily
run the `go test` incantation for integration tests itself, just as it does
for `make test`, and it is immediately different how integration tests are
different from normal ones.

Also update some related documentation.

Author: petr-muller

Makefile

@@ -12,7 +12,8 @@ test:
 .PHONY: test
 
 integration-test:
-	hack/test-integration.sh
+	# This runs against a real OpenShift cluster through the current KUBECONFIG context
+	TEST_INTEGRATION=1 go test ./... -test.run=^TestIntegration
 .PHONY: integration-test
 
 update: build

docs/dev/README.md

@@ -59,13 +59,12 @@ Run `make test` to execute all unit tests.
 
 ### Integration tests
 
-The [`hack/test-integration.sh`](../../hack/test-integration.sh) script executes integration tests against an OpenShift
-cluster. It requires `$KUBECONFIG` with administrator credentials. Only execute integration tests against disposable
-testing clusters.
+Run `make integration-test` to execute integration tests against an OpenShift cluster. It requires `$KUBECONFIG` with
+administrator credentials. Only execute integration tests against disposable testing clusters.
 
 ```console
 $ export KUBECONFIG=<admin kubeconfig path>
-$ ./hack/test-integration.sh
+$ make integration-test
 ```
 
 ### Replace cluster's CVO with development CVO release payload image