Validate plugin has redundant code for "replace_tokens" field

Problem/Motivation

In \Drupal\eca\Entity\Eca::validatePlugin we recognize all boolean fields and check if we have to convert back the yes/no dropdown values to boolean at the beginning. Right afterwards, we separately check for replace_tokens fields and see if we have to convert them as well.

However, we've changed all those fields into checkboxes and therefore they are already covered by the first conversion. We can remove that code block completely.