Fix mistake in _remove_theme_attribute_in_block_template_content

[talldan]

Description

In #36898 I tried updating the _remove_theme_attribute_in_block_template_content to match core, but made a mistake in that I only copy/pasted the if statement over from core (https://github.com/WordPress/gutenberg/pull/36898/files#r762692470).

The part that I missed is that in WordPress core, the condition of the if statement was inverted. I should've updated the whole function.

In this PR I'm fixing that.

Thanks to @noisysocks for spotting the issue.

How has this been tested?

I don't really know how to test this, but the function as defined in this PR should now match WordPress core:
https://github.com/WordPress/wordpress-develop/blob/7f1166c8918b4862fd1b8dd72ba6bf5b723b369e/src/wp-includes/block-template-utils.php#L466-L488

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[noisysocks]

It now looks the same as it does in Core! 👍

https://github.com/WordPress/wordpress-develop/blob/7f1166c8918b4862fd1b8dd72ba6bf5b723b369e/src/wp-includes/block-template-utils.php#L466-L488