chore: remove monocdk branch workflows [skip ci] (#162)

Followup to #158. Since the workflow's getting executed on the `main` branch, I opted to simply remove the workflows. This should stop the weekly updates to the unsupported monocdk version.

If for some reason they're needed again, we can revert this change.

---

_By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license_

Author: Eugene Cheung

.gitattributes

@@ -12,11 +12,6 @@ const project = new awscdk.AwsCdkConstructLibrary({
 
   defaultReleaseBranch: "main",
   majorVersion: 1,
-  releaseBranches: {
-    monocdk: {
-      majorVersion: 0,
-    },
-  },
   stability: "experimental",
 
   cdkVersion: CDK_VERSION,