Backport [PR 18772] Remove unnecesary "header" block redeclaration

[samuel27m]

Description (*)

Same as pull request #18772
Removed because the "header" block was already created in module Magento_Theme, so this redeclaration is just unnecesary

Fixed Issues (if relevant)

None found on the Magento 2 issues

Manual testing scenarios (*)

1. Create new module.

2. Create new default.xml and try to use referenceBlock to set the template. Example:
   <referenceBlock name="header"> <arguments> <argument name="template" xsi:type="string">Samuel27m_Welcome::html/header.phtml</argument> </arguments> </referenceBlock>

3. Doesn't work, because Luma theme XML files are compiled after all modules and overrides the custom module modifications.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @samuel27m. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me $VERSION instance - deploy vanilla Magento instance

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[magento-engcom-team]

Hi @VladimirZaets, thank you for the review.
ENGCOM-3341 has been created to process this Pull Request

[magento-engcom-team]

Hi @samuel27m. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.