MQE-1224: Remove fillSecretField from Test Schema

[tomreece]

Description

• Removed fillSecretField from test schema

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/verification tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)
• Changes to Framework doesn't have backward incompatible changes for tests or have related Pull Request with fixes to tests

[coveralls]

Coverage decreased (-1.5%) to 55.443% when pulling e2b7068 on MQE-1224 into f8cf2c3 on develop.

[tomreece]

I believe this change is backwards compatible and does not need to wait for a major version.

The fillSecretField action could not have be used without throwing an error message (see https://github.com/magento/magento2-functional-testing-framework/blob/develop/src/Magento/FunctionalTestingFramework/Test/Util/ActionMergeUtil.php#L112)

[KevinBKozan]

@tomreece You are correct for the most part.
So since per-file schema validation isn't run by default, you are correct in that it explicitly does not break generation. You can use <fillSecretField> just like you can use <whateveractionhere>, the generator does not whitelist actions to an explicit list, the schema does that validation.

There isn't a good reason for anyone to be using fillSecretField due to what you pointed out, and evern if someone had created a test using it, normal generation isn't broken.