Remove the dead include of the removed includes/helpers.inc from varbase_media_header.install

Problem/Motivation

varbase_media_header.install line 12 does include_once __DIR__ . '/includes/helpers.inc';, but includes/helpers.inc was removed from the package. On drush updatedb (and install) this logs a harmless PHP include warning (include_once(...helpers.inc): Failed to open stream).

Same class of bug as the earlier Varbase Auth helpers.inc fix (#3609191).

Steps to reproduce

1. Install or run database updates on a site with Varbase Media Header 9.2.0 / 10.1.1.
2. Observe an include warning for the missing includes/helpers.inc.

Proposed resolution

Delete the dead include_once ... '/includes/helpers.inc'; line (keep the includes/updates.inc include). Ship on both the 9.2.x and 10.1.x branches.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ❌ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ✅ Update Release Notes
• ✅ Release: varbase_media_header-10.1.2, varbase_media_header-9.2.1

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• Removed a dead include_once of the removed includes/helpers.inc in varbase_media_header.install that logged a harmless include warning on update/install.