Remove manually created extension interface

[sivaschenko]

Description

Even though extension attributes PHP classes/interfaces were not supposed to be created manually, we have several such occurrences.
It is preferable to remove these classes/interfaces because they block extensibility of the base interfaces.

Details

Remove Magento\PageBuilder\Api\Data\TemplateExtensionInterface for 2.5 release line

[m2-assistant]

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