chore(ci): remove mastheadv2 jobs now that its fully released on v2 (#…

…12132)

### Description

Cleanup task. CI has been [failing for awhile for the automerge-mastheadv2 job](https://github.com/carbon-design-system/carbon-for-ibm-dotcom/actions/workflows/automerge-mastheadv2.yml). This job was responsible for merging `main` back to the `feat/masthead-v2-dev` whenever `main` was updated. This is no longer necessary as we don't use the `feat/masthead-v2` branch, and it's now quite behind. It fails b/c of merge conclicts there.

### Changelog

**Changed**

- Avoid running e2e-integration test suite on `feat/masthead-*` branches as they are no longer used.

**Removed**

- Removed the `automerge-mastheadv2` job

<!-- React and Web Component deploy previews are enabled by default. -->
<!-- To enable additional available deploy previews, apply the following -->
<!-- labels for the corresponding package: -->
<!-- *** "test: e2e": Codesandbox examples and e2e integration tests -->
<!-- *** "package: services": Services -->
<!-- *** "package: utilities": Utilities -->
<!-- *** "RTL": React / Web Components (RTL) -->
<!-- *** "feature flag": React / Web Components (experimental) -->

.github/workflows/e2e-integration.yml

@@ -2,7 +2,7 @@ name: e2e-integration
 
 on:
   push:
-    branches: [ main, release/v2*, feat/masthead-v2, feat/masthead-v2-dev ]
+    branches: [ main, release/v2* ]
   schedule:
     - cron: '0 20 * * 1-5'