Remove unneeded replace pin for sigs.k8s.io/structured-merge-diff

Update the root go.mod and remove the replace pin for
sigs.k8s.io/structured-merge-diff.

It looks like the "// pinned because no tag supports 1.18 yet" message
may have also been an artifact of removing the helm replace pin that was
previously in place.

Author: timflannagan

Diff for: go.mod

@@ -64,7 +64,4 @@ replace (
 	// Patch for a race condition involving metadata-only
 	// informers until it can be resolved upstream:
 	sigs.k8s.io/controller-runtime v0.9.2 => github.com/benluddy/controller-runtime v0.9.3-0.20210720171926-9bcb99bd9bd3
-
-	// pinned because no tag supports 1.18 yet
-	sigs.k8s.io/structured-merge-diff => sigs.k8s.io/structured-merge-diff v1.0.1-0.20191108220359-b1b620dd3f06
 )

Diff for: vendor/modules.txt

@@ -1861,4 +1861,3 @@ sigs.k8s.io/structured-merge-diff/v4/value
 sigs.k8s.io/yaml
 # github.com/openshift/api => github.com/openshift/api v0.0.0-20200331152225-585af27e34fd
 # github.com/openshift/client-go => github.com/openshift/client-go v0.0.0-20200326155132-2a6cd50aedd0
-# sigs.k8s.io/structured-merge-diff => sigs.k8s.io/structured-merge-diff v1.0.1-0.20191108220359-b1b620dd3f06