cache_fallback_discovery_client_test.go: Hard-code version

I'm not sure what to do with this test.  Install stream data has
moved to RP-Config, so if the test is worth keeping then I guess
the oldest supported version will have to be hard-coded and kept
up-to-date.  But it probably won't be.

pkg/util/dynamichelper/discovery/cache_fallback_discovery_client_test.go

@@ -19,7 +19,6 @@ import (
 
 	"github.com/Azure/ARO-RP/pkg/util/cmp"
 	utillog "github.com/Azure/ARO-RP/pkg/util/log"
-	"github.com/Azure/ARO-RP/pkg/util/version"
 	utilerror "github.com/Azure/ARO-RP/test/util/error"
 )
 
@@ -37,7 +36,7 @@ func TestVersion(t *testing.T) {
 
 	assetsVersion := strings.TrimSuffix(string(b), "\n")
 	// NOTE: This is checking for the version of the oldest supported minor stream.
-	if assetsVersion != version.DefaultInstallStreams[10].Version.String() {
+	if assetsVersion != "4.10.63" {
 		t.Error("discovery cache is out of date: run make discoverycache")
 	}
 }