docs: cleanup aftert test should remove namespace, Closes #6791

[m-szalik]

Two examples have the same issue. Should be: namespace should be removed at the end of the test It is: namespace is created again

• https://github.com/operator-framework/operator-sdk/blob/e95abdbd5ccb7ca0fd586e0c6f578e491b0a025b/testdata/go/v4/memcached-operator/test/e2e/e2e_test.go#L87
• https://github.com/operator-framework/operator-sdk/blob/e95abdbd5ccb7ca0fd586e0c6f578e491b0a025b/testdata/go/v4/monitoring/memcached-operator/test/e2e/e2e_test.go#L103