changefeedccl: future-proof TestChangefeedBackfillCheckpoint

This testing knob was added in #68374 but I'm not sure that it was
necessary? Brief stress runs with an without this flag did not surface
anything. In a future commit where we enable span configs by default,
we'll actually rely on the reconciliation job running, so we get rid of
this flag now.

Release note: None

pkg/ccl/changefeedccl/BUILD.bazel

@@ -177,7 +177,6 @@ go_test(
         "//pkg/server/status",
         "//pkg/server/telemetry",
         "//pkg/settings/cluster",
-        "//pkg/spanconfig",
         "//pkg/sql",
         "//pkg/sql/catalog",
         "//pkg/sql/catalog/catalogkv",

pkg/ccl/changefeedccl/helpers_test.go

@@ -34,7 +34,6 @@ import (
 	"github.com/cockroachdb/cockroach/pkg/jobs"
 	"github.com/cockroachdb/cockroach/pkg/security"
 	"github.com/cockroachdb/cockroach/pkg/server"
-	"github.com/cockroachdb/cockroach/pkg/spanconfig"
 	"github.com/cockroachdb/cockroach/pkg/sql/execinfra"
 	"github.com/cockroachdb/cockroach/pkg/sql/sem/tree"
 	"github.com/cockroachdb/cockroach/pkg/testutils"
@@ -329,7 +328,6 @@ func startTestFullServer(
 	knobs := base.TestingKnobs{
 		DistSQL:          &execinfra.TestingKnobs{Changefeed: &TestingKnobs{}},
 		JobsTestingKnobs: jobs.NewTestingKnobsWithShortIntervals(),
-		SpanConfig:       &spanconfig.TestingKnobs{ManagerDisableJobCreation: true},
 	}
 	if options.knobsFn != nil {
 		options.knobsFn(&knobs)