fix: do not forcefully set area in template if it is already set, fixes #14968, fixes #13530

[mageprince]

Original Pull Request

#15137

Description

This reverts the addition of the enforced area in d3aef7c which caused #14968 and introduced this regression

Fixed Issues (if relevant)

1. Can't change the applied theme in 2.2.4 #14968: Can't change the applied theme in 2.2.4
2. "Template file 'header.html' is not found" error while trying to save Design Configuration. #13530: "Template file 'header.html' is not found" error while trying to save Design Configuration.

Manual testing scenarios

1. comment / disable line 117 in app/code/Magento/Theme/Model/Design/Config/Validator.php
2. change theme in store view

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]

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[DanielRuf]

Did you test if this is needed in 2.3? It does not seem so as there is no exception thrown.

[DanielRuf]

@magento-engcom-team give me 2.3-develop instance

[magento-engcom-team]

Hi @DanielRuf. Thank you for your request. I'm working on Magento 2.3-develop instance for you

[magento-engcom-team]

Hi @DanielRuf, here is your Magento instance.
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[DanielRuf]

Not reproducible in 2.3.