Remove unneeded replace pin for sigs.k8s.io/structured-merge-diff (#2338)

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.

Signed-off-by: timflannagan <[email protected]>
Upstream-repository: operator-lifecycle-manager
Upstream-commit: 3eb0d7be413d7c185441cb3fc81dadb2b67706e7

Author: timflannagan

staging/operator-lifecycle-manager/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
 )